feat(FN-3406): add shell context hook plumbing for dashboard

Adds a new `useShellContext` hook with tests to the dashboard, integrated into `App.tsx` and `Header.tsx` to provide shell context plumbing throughout the UI.

Fusion-Task-Id: FN-3406

packages/dashboard/app/hooks/__tests__/useShellContext.test.ts

@@ -0,0 +1,56 @@
+import { describe, expect, it, vi } from "vitest";
+import { renderHook } from "@testing-library/react";
+import { useShellContext } from "../useShellContext";
+
+const mockUseNativeShellContext = vi.fn();
+
+vi.mock("../../context/ShellContext", () => ({
+  useShellContext: () => mockUseNativeShellContext(),
+}));
+
+describe("useShellContext", () => {
+  it("returns shell context from URL handoff when present", () => {
+    window.history.replaceState({}, "", "/?shellKind=desktop&shellMode=remote&profileId=p1&serverBaseUrl=https://remote.example.com");
+    mockUseNativeShellContext.mockReturnValue({
+      state: { host: "web", activeProfileId: null, profiles: [] },
+    });
+
+    const { result } = renderHook(() => useShellContext());
+
+    expect(result.current.shellContext).toMatchObject({
+      shellKind: "desktop",
+      shellMode: "remote",
+      profileId: "p1",
+      serverBaseUrl: "https://remote.example.com",
+    });
+    expect(result.current.isDesktopShell).toBe(true);
+  });
+
+  it("falls back to normalized desktop local shell context", () => {
+    window.history.replaceState({}, "", "/");
+    mockUseNativeShellContext.mockReturnValue({
+      state: { host: "desktop-shell", desktopMode: "local", activeProfileId: null, profiles: [] },
+    });
+
+    const { result } = renderHook(() => useShellContext());
+
+    expect(result.current.shellContext).toEqual({
+      shellKind: "desktop",
+      shellMode: "local",
+      capabilities: { canOpenConnectionManager: true },
+    });
+  });
+
+  it("returns null context in plain browser sessions", () => {
+    window.history.replaceState({}, "", "/");
+    mockUseNativeShellContext.mockReturnValue({
+      state: { host: "web", activeProfileId: null, profiles: [] },
+    });
+
+    const { result } = renderHook(() => useShellContext());
+
+    expect(result.current.shellContext).toBeNull();
+    expect(result.current.isDesktopShell).toBe(false);
+    expect(result.current.isMobileShell).toBe(false);
+  });
+});