FN-5884: preserve quick chat session across reopens

Keep quick chat warm and restore the last opened conversation per project.

- persist the active quick chat session id in per-project local storage and restore it before falling back to latest activity
- retain in-memory quick chat state across close/reopen while resetting chat state correctly when the project changes
- add storage, hook, and FAB coverage for restored sessions, warm reopen behavior, and project switching; document the updated quick chat behavior

Files changed:
 docs/dashboard-guide.md                            |   3 +-
 packages/dashboard/app/components/QuickChatFAB.tsx |  29 ++++--
 .../app/components/__tests__/QuickChatFAB.test.tsx | 112 +++++++++++++++++++--
 .../__tests__/quickChatLastSessionStorage.test.ts  |  55 ++++++++++
 .../app/hooks/__tests__/useQuickChat.test.ts       |  70 +++++++++++++
 .../app/hooks/quickChatLastSessionStorage.ts       |  41 ++++++++
 packages/dashboard/app/hooks/useQuickChat.ts       |  25 +++++
 7 files changed, 319 insertions(+), 16 deletions(-)

Fusion-Task-Id: FN-5884

Fusion-Task-Lineage: 4d689611-1824-41db-8fc2-d543407316ff

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

@@ -0,0 +1,55 @@
+import { beforeEach, describe, expect, it, vi } from "vitest";
+import {
+  getPersistedLastQuickChatSessionId,
+  removePersistedLastQuickChatSessionId,
+  setPersistedLastQuickChatSessionId,
+} from "../quickChatLastSessionStorage";
+
+describe("quickChatLastSessionStorage", () => {
+  beforeEach(() => {
+    localStorage.clear();
+    vi.restoreAllMocks();
+  });
+
+  it("stores and retrieves the last quick chat session id per project", () => {
+    setPersistedLastQuickChatSessionId("proj-123", "session-123");
+
+    expect(getPersistedLastQuickChatSessionId("proj-123")).toBe("session-123");
+    expect(localStorage.getItem("fusion:quick-chat-last-session:proj-123")).toBe("session-123");
+  });
+
+  it("uses a default storage bucket when project id is missing", () => {
+    setPersistedLastQuickChatSessionId(undefined, "session-default");
+
+    expect(getPersistedLastQuickChatSessionId()).toBe("session-default");
+    expect(localStorage.getItem("fusion:quick-chat-last-session:default")).toBe("session-default");
+  });
+
+  it("removes persisted session ids per project", () => {
+    setPersistedLastQuickChatSessionId("proj-123", "session-123");
+
+    removePersistedLastQuickChatSessionId("proj-123");
+
+    expect(getPersistedLastQuickChatSessionId("proj-123")).toBeNull();
+  });
+
+  it("returns null when nothing is saved", () => {
+    expect(getPersistedLastQuickChatSessionId("proj-123")).toBeNull();
+  });
+
+  it("swallows localStorage failures", () => {
+    vi.spyOn(Storage.prototype, "setItem").mockImplementation(() => {
+      throw new Error("quota exceeded");
+    });
+    vi.spyOn(Storage.prototype, "getItem").mockImplementation(() => {
+      throw new Error("blocked");
+    });
+    vi.spyOn(Storage.prototype, "removeItem").mockImplementation(() => {
+      throw new Error("blocked");
+    });
+
+    expect(() => setPersistedLastQuickChatSessionId("proj-123", "session-123")).not.toThrow();
+    expect(getPersistedLastQuickChatSessionId("proj-123")).toBeNull();
+    expect(() => removePersistedLastQuickChatSessionId("proj-123")).not.toThrow();
+  });
+});