feat(FN-2443): unify plugin settings into tabbed Plugins section

- Remove the standalone Pi Extensions section from settings navigation and route legacy initialSection values to Plugins
- Add accessible subsection tabs in Plugins for Fusion Plugins and Pi Extensions with proper tab/tabpanel semantics
- Render plugin content conditionally per active subsection so only the selected manager is mounted
- Add SettingsModal tests covering sidebar navigation, tab accessibility, and subsection switching behavior
- Add subsection styling in SettingsModal.css and preserve extracted styles.css boundaries

packages/dashboard/app/components/SettingsModal.css

@@ -150,6 +150,52 @@
   margin-top: var(--space-xl);
 }
 
+.settings-plugins-subsection-toggle {
+  display: inline-flex;
+  gap: var(--space-xs);
+  padding: 0 var(--space-xl);
+  margin: var(--space-md) 0;
+}
+
+.settings-plugins-subsection-btn {
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+  border: var(--btn-border-width) solid var(--border);
+  border-radius: var(--radius-pill);
+  background: var(--surface);
+  color: var(--text-muted);
+  font-size: 0.85rem;
+  font-weight: 600;
+  padding: var(--space-xs) var(--space-md);
+  cursor: pointer;
+  transition:
+    background var(--transition-fast),
+    color var(--transition-fast),
+    border-color var(--transition-fast),
+    box-shadow var(--transition-fast);
+}
+
+.settings-plugins-subsection-btn:hover {
+  background: var(--surface-hover);
+  color: var(--text);
+}
+
+.settings-plugins-subsection-btn:focus-visible {
+  outline: none;
+  box-shadow: var(--focus-ring-strong);
+}
+
+.settings-plugins-subsection-btn.active {
+  background: color-mix(in srgb, var(--todo) 14%, transparent);
+  border-color: color-mix(in srgb, var(--todo) 45%, var(--border));
+  color: var(--todo);
+}
+
+.settings-plugins-subsection-panel {
+  padding-bottom: var(--space-md);
+}
+
 /* Scope indicators in sidebar nav items */
 .settings-scope-icon {
   margin-right: 6px;
@@ -676,6 +722,17 @@
     margin: 0 var(--space-lg) 0;
   }
 
+  .settings-plugins-subsection-toggle {
+    display: flex;
+    width: 100%;
+    padding: 0 var(--space-lg);
+  }
+
+  .settings-plugins-subsection-btn {
+    flex: 1;
+    min-height: 36px;
+  }
+
   .form-group {
     padding: 0 14px;
   }

packages/dashboard/app/components/SettingsModal.tsx

@@ -29,7 +29,7 @@ import "./SettingsModal.css";
  *
  * Group headers (isGroupHeader: true) are non-clickable labels that visually group sections.
  * The sidebar is organized into three groups:
- *   - Account: Scope-less sections (authentication, pi-extensions)
+ *   - Account: Scope-less sections (authentication)
  *   - Global: Global-scoped sections (appearance, notifications, node-sync, global-models)
  *   - Project: Project-scoped sections (project-models, general, scheduling, worktrees, commands,
  *     merge, memory, experimental, prompts, backups, plugins)
@@ -54,7 +54,6 @@ const SETTINGS_SECTIONS: SettingsSection[] = [
   // Account group (scope-less items — independent of settings storage)
   { id: "__account_header", label: "Account", scope: undefined, isGroupHeader: true },
   { id: "authentication", label: "Authentication", scope: undefined, icon: Globe },
-  { id: "pi-extensions", label: "Pi Extensions", scope: undefined },
 
   // Global group (shared across all Fusion projects)
   { id: "__global_header", label: "Global", scope: undefined, isGroupHeader: true },
@@ -102,7 +101,10 @@ const KNOWN_EXPERIMENTAL_FEATURES: Record<string, string> = {
   devServerView: "Dev Server View",
 };
 
-export type SectionId = SettingsSection["id"];
+type LegacySectionId = "pi-extensions";
+export type SectionId = SettingsSection["id"] | LegacySectionId;
+
+type PluginsSubsectionId = "fusion-plugins" | "pi-extensions";
 
 /** Local form state extends Settings with a worktreeInitCommand override and lets tokenCap carry null (delete semantic). */
 type SettingsFormState = Settings & { worktreeInitCommand?: string; tokenCap?: number | null };
@@ -162,7 +164,17 @@ export function SettingsModal({
   const [initialScopedValues, setInitialScopedValues] = useState<{ global: GlobalSettings; project: Partial<Settings> } | null>(null);
   // Find the first non-group-header section for default active section
   const firstNonHeaderSection = SETTINGS_SECTIONS.find((s) => !s.isGroupHeader);
-  const [activeSection, setActiveSection] = useState<SectionId>(initialSection ?? firstNonHeaderSection?.id ?? "authentication");
+  const [activeSection, setActiveSection] = useState<SectionId>(() => {
+    if (initialSection === "pi-extensions") {
+      return "plugins";
+    }
+    return initialSection ?? firstNonHeaderSection?.id ?? "authentication";
+  });
+  // Deterministic default: opening Plugins starts on Fusion Plugins unless legacy
+  // `initialSection="pi-extensions"` is explicitly provided.
+  const [activePluginsSubsection, setActivePluginsSubsection] = useState<PluginsSubsectionId>(() =>
+    initialSection === "pi-extensions" ? "pi-extensions" : "fusion-plugins",
+  );
   const [showMobileSectionPicker, setShowMobileSectionPicker] = useState(() =>
     typeof window !== "undefined" && typeof window.matchMedia === "function"
       ? window.matchMedia(MOBILE_SETTINGS_MEDIA_QUERY)?.matches === true
@@ -3110,18 +3122,63 @@ export function SettingsModal({
           <>
             {renderScopeBanner()}
             <h4 className="settings-section-heading">Plugins</h4>
-            <Suspense fallback={null}>
-              <PluginManager addToast={addToast} projectId={projectId} />
-            </Suspense>
-            <PluginSlot slotId="settings-section" projectId={projectId} />
+            <div className="settings-plugins-subsection-toggle" role="tablist" aria-label="Plugin manager type">
+              <button
+                type="button"
+                id="plugins-tab-fusion-plugins"
+                role="tab"
+                aria-controls="plugins-panel-fusion-plugins"
+                aria-selected={activePluginsSubsection === "fusion-plugins"}
+                tabIndex={activePluginsSubsection === "fusion-plugins" ? 0 : -1}
+                className={`settings-plugins-subsection-btn${activePluginsSubsection === "fusion-plugins" ? " active" : ""}`}
+                onClick={() => setActivePluginsSubsection("fusion-plugins")}
+              >
+                Fusion Plugins
+              </button>
+              <button
+                type="button"
+                id="plugins-tab-pi-extensions"
+                role="tab"
+                aria-controls="plugins-panel-pi-extensions"
+                aria-selected={activePluginsSubsection === "pi-extensions"}
+                tabIndex={activePluginsSubsection === "pi-extensions" ? 0 : -1}
+                className={`settings-plugins-subsection-btn${activePluginsSubsection === "pi-extensions" ? " active" : ""}`}
+                onClick={() => setActivePluginsSubsection("pi-extensions")}
+              >
+                Pi Extensions
+              </button>
+            </div>
+            <div
+              id="plugins-panel-fusion-plugins"
+              role="tabpanel"
+              aria-labelledby="plugins-tab-fusion-plugins"
+              className="settings-plugins-subsection-panel"
+              hidden={activePluginsSubsection !== "fusion-plugins"}
+            >
+              {activePluginsSubsection === "fusion-plugins" && (
+                <>
+                  <Suspense fallback={null}>
+                    <PluginManager addToast={addToast} projectId={projectId} />
+                  </Suspense>
+                  <PluginSlot slotId="settings-section" projectId={projectId} />
+                </>
+              )}
+            </div>
+            <div
+              id="plugins-panel-pi-extensions"
+              role="tabpanel"
+              aria-labelledby="plugins-tab-pi-extensions"
+              className="settings-plugins-subsection-panel"
+              hidden={activePluginsSubsection !== "pi-extensions"}
+            >
+              {activePluginsSubsection === "pi-extensions" && (
+                <Suspense fallback={null}>
+                  <PiExtensionsManager addToast={addToast} projectId={projectId} />
+                </Suspense>
+              )}
+            </div>
           </>
         );
-      case "pi-extensions":
-        return (
-          <Suspense fallback={null}>
-            <PiExtensionsManager addToast={addToast} projectId={projectId} />
-          </Suspense>
-        );
       case "authentication": {
         // CLI-backed providers (currently just claude-cli) render their own
         // compact card with Enable/Disable + Test actions — bypassing the

packages/dashboard/app/components/__tests__/SettingsModal.test.tsx

@@ -1,5 +1,5 @@
 import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";
-import { render, screen, fireEvent, waitFor } from "@testing-library/react";
+import { render, screen, fireEvent, waitFor, within } from "@testing-library/react";
 import userEvent from "@testing-library/user-event";
 import { SettingsModal } from "../SettingsModal";
 import type { SettingsExportData } from "../../api";
@@ -61,6 +61,18 @@ vi.mock("../../hooks/useMemoryBackendStatus", () => ({
   useMemoryBackendStatus: (...args: unknown[]) => mockUseMemoryBackendStatus(...args),
 }));
 
+vi.mock("../PluginManager", () => ({
+  PluginManager: () => <div data-testid="plugin-manager">Plugin manager content</div>,
+}));
+
+vi.mock("../PiExtensionsManager", () => ({
+  PiExtensionsManager: () => <div data-testid="pi-extensions-manager">Pi extensions content</div>,
+}));
+
+vi.mock("../PluginSlot", () => ({
+  PluginSlot: () => <div data-testid="plugin-slot">Plugin slot content</div>,
+}));
+
 const noop = () => {};
 
 const defaultSettings = {
@@ -90,6 +102,13 @@ function renderModal(props = {}) {
   );
 }
 
+async function waitForSettingsModalReady() {
+  await waitFor(() => {
+    expect(mockFetchSettings).toHaveBeenCalled();
+    expect(screen.queryByText("Loading…")).not.toBeInTheDocument();
+  });
+}
+
 describe("SettingsModal", () => {
   beforeEach(() => {
     vi.clearAllMocks();
@@ -318,6 +337,55 @@ describe("SettingsModal", () => {
     });
   });
 
+  describe("Plugins section navigation", () => {
+    it("does not render a standalone Pi Extensions sidebar item", async () => {
+      renderModal();
+      await waitForSettingsModalReady();
+
+      const sidebar = document.querySelector(".settings-sidebar");
+      expect(sidebar).toBeInTheDocument();
+      expect(within(sidebar as HTMLElement).queryByRole("button", { name: /Pi Extensions$/ })).not.toBeInTheDocument();
+      expect(within(sidebar as HTMLElement).getByRole("button", { name: /Plugins$/ })).toBeInTheDocument();
+    });
+
+    it("renders accessible tab semantics for Plugins subsection controls", async () => {
+      renderModal();
+      await waitForSettingsModalReady();
+
+      await userEvent.click(await screen.findByRole("button", { name: /Plugins$/ }));
+
+      const tablist = await screen.findByRole("tablist", { name: "Plugin manager type" });
+      const fusionTab = within(tablist).getByRole("tab", { name: "Fusion Plugins" });
+      const piTab = within(tablist).getByRole("tab", { name: "Pi Extensions" });
+
+      expect(fusionTab).toHaveAttribute("aria-selected", "true");
+      expect(fusionTab).toHaveAttribute("aria-controls", "plugins-panel-fusion-plugins");
+      expect(piTab).toHaveAttribute("aria-selected", "false");
+      expect(piTab).toHaveAttribute("aria-controls", "plugins-panel-pi-extensions");
+    });
+
+    it("switches between Fusion Plugins and Pi Extensions managers", async () => {
+      renderModal();
+      await waitForSettingsModalReady();
+
+      await userEvent.click(await screen.findByRole("button", { name: /Plugins$/ }));
+
+      expect(await screen.findByTestId("plugin-manager")).toBeInTheDocument();
+      expect(screen.getByTestId("plugin-slot")).toBeInTheDocument();
+      expect(screen.queryByTestId("pi-extensions-manager")).not.toBeInTheDocument();
+
+      await userEvent.click(screen.getByRole("tab", { name: "Pi Extensions" }));
+
+      await waitFor(() => {
+        expect(screen.getByTestId("pi-extensions-manager")).toBeInTheDocument();
+      });
+      expect(screen.queryByTestId("plugin-manager")).not.toBeInTheDocument();
+      expect(screen.queryByTestId("plugin-slot")).not.toBeInTheDocument();
+      expect(screen.getByRole("tabpanel", { name: "Pi Extensions" })).toBeVisible();
+      expect(document.getElementById("plugins-panel-fusion-plugins")).toHaveAttribute("hidden");
+    });
+  });
+
   describe("Number input clearing", () => {
     it("allows clearing maxConcurrent without leaving a stuck zero", async () => {
       renderModal();