feat(FN-3344): preserve history state in deep-link close

Fixes deep-link close to preserve browser history state and adds comprehensive integration tests for the navigation history feature, covering App component and useDeepLink hook behavior.

Fusion-Task-Id: FN-3344

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

@@ -936,7 +936,7 @@ describe("App deep link handling", () => {
     // Should have cleaned the task param from the URL via replaceState
     await waitFor(() => {
       expect(window.history.replaceState).toHaveBeenCalledWith(
-        null,
+        expect.any(Object),
         "",
         "/",
       );
@@ -972,7 +972,7 @@ describe("App deep link handling", () => {
     // Should have removed only the task param, keeping project param
     await waitFor(() => {
       expect(window.history.replaceState).toHaveBeenCalledWith(
-        null,
+        expect.any(Object),
         "",
         "/?project=proj_456",
       );

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

@@ -142,11 +142,15 @@ describe("useDeepLink", () => {
     });
   });
 
-  it("cleans task query param when deep-linked modal closes", async () => {
+  it("cleans task query param when deep-linked modal closes and preserves history state", async () => {
     Object.defineProperty(window, "location", {
       configurable: true,
       value: new URL("http://localhost:3000/?project=proj_123&task=FN-123"),
     });
+    const replaceStateMock = window.history.replaceState as ReturnType<typeof vi.fn>;
+    window.history.replaceState = originalReplaceState;
+    window.history.replaceState({ navIndex: 2, existing: "value" }, "");
+    window.history.replaceState = replaceStateMock;
 
     const { result, closeTaskDetail } = renderUseDeepLink();
 
@@ -157,7 +161,7 @@ describe("useDeepLink", () => {
     result.current.handleDetailClose();
 
     expect(window.history.replaceState).toHaveBeenCalledWith(
-      null,
+      { navIndex: 2, existing: "value" },
       "",
       "/?project=proj_123",
     );

packages/dashboard/app/hooks/useDeepLink.ts

@@ -91,8 +91,9 @@ export function useDeepLink(options: UseDeepLinkOptions): UseDeepLinkResult {
       const params = new URLSearchParams(window.location.search);
       params.delete("task");
       const query = params.toString();
+      const existingState = window.history.state ?? {};
       window.history.replaceState(
-        null,
+        existingState,
         "",
         query ? `${window.location.pathname}?${query}` : window.location.pathname,
       );