refactor(nav): drop redundant sub-view markDirty; add useFilterState re-hydrate tests

Addresses the 2 findings from /code-review high on 726c382:
- NavigationProvider: openChannel/enterYtSearch no longer markDirty. A sub-view is an
  overlay, not a page change — the page re-hydrate's replaceState already spreads the
  _chan/_yt marker, so the overlay survives on top of the RESTORED page. Removing the
  markDirty lets a fresh tab still restore the stored landing page under a pre-pin overlay
  (previously it stuck on the default). setPage keeps markDirty (a real navigation).
- useFilterState.dom.test.ts: model a mutable pinned account and add 3 integration tests —
  null->id re-hydrate adopts the stored map, the migrate seed carries a legacy value forward
  on re-hydrate when the key is empty, and a pre-pin edit is preserved (markDirty guard).

gate-green: vite build + vitest 120 + eslint-0 + knip + prettier. Fresh-tab restore re-E2E'd
(lands on Plex with full state, zero console errors).

frontend/src/components/NavigationProvider.tsx

@@ -115,8 +115,8 @@ export function NavigationProvider({ children }: { children: ReactNode }) {
     setPageState(stored);
     setAccountRaw(PAGE_KEY, stored); // no-op re-write, keeps storage authoritative
     // Correct the CURRENT entry (replace, not push) and PRESERVE any markers already on it — a channel
-    // (_chan) or YT search (_yt) opened in the pre-pin window must survive, or its overlay is orphaned
-    // from history and Back can't close it. (Those setters also markDirty, so this usually no-ops then.)
+    // (_chan) or YT search (_yt) opened in the pre-pin window survives on top of the restored page,
+    // rather than being orphaned from history so Back can't close it.
     window.history.replaceState({ ...(window.history.state || {}), sfPage: stored }, "");
   });
 
@@ -137,25 +137,23 @@ export function NavigationProvider({ children }: { children: ReactNode }) {
     [markDirty],
   );
 
-  const openChannel = useCallback(
-    (id: string, name?: string) => {
-      markDirty(); // opening a sub-view is a navigation — don't let a late account-pin re-hydrate override the page
+  // A sub-view (channel / YT search) is an OVERLAY, not a page change, so it does NOT markDirty: a
+  // pre-pin account re-hydrate should still restore the stored page underneath — the re-hydrate's
+  // replaceState spreads the _chan/_yt markers, so the overlay survives on top of the restored page.
+  const openChannel = useCallback((id: string, name?: string) => {
     setChannelView({ id, name });
     const st = window.history.state || {};
     if (st._chan) window.history.replaceState({ ...st, _chan: id, _chanName: name ?? null }, "");
     else window.history.pushState({ ...st, _chan: id, _chanName: name ?? null }, "");
-    },
-    [markDirty],
-  );
+  }, []);
 
   const closeChannel = useCallback(() => {
     if (window.history.state?._chan) window.history.back();
     else setChannelView(null);
   }, []);
 
-  const enterYtSearch = useCallback(
-    (q: string) => {
-      markDirty(); // a sub-view navigation — see openChannel
+  const enterYtSearch = useCallback((q: string) => {
+    // Overlay, not a page change — see openChannel: no markDirty, the re-hydrate preserves _yt.
     setYtSearch(q);
     // Drop any prior _ytScrape marker — the new search's source isn't known yet; Feed re-stamps it
     // once the results resolve, so a reload only restores a confirmed scrape-sourced search.
@@ -163,9 +161,7 @@ export function NavigationProvider({ children }: { children: ReactNode }) {
     if (st._yt)
       window.history.replaceState({ ...st, _yt: q }, ""); // refine current search
     else window.history.pushState({ ...st, sfPage: "feed", _yt: q }, ""); // new sub-view entry
-    },
-    [markDirty],
-  );
+  }, []);
 
   const exitYtSearch = useCallback(() => {
     // Pop our search entry (so Forward still works); fall back to a plain clear if we don't own one.

frontend/src/lib/useFilterState.dom.test.ts

@@ -6,21 +6,27 @@
 import { describe, it, expect, beforeEach, vi } from "vitest";
 import { renderHook, act } from "@testing-library/react";
 
-const { store } = vi.hoisted(() => ({ store: new Map<string, string>() }));
+// `ctl.account` models the tab's pinned account: getAccountRaw returns null while it's null (mirrors the
+// real accountKey → no key → null), so a fresh-tab mount reads defaults until the account is pinned.
+const { store, ctl } = vi.hoisted(() => ({
+  store: new Map<string, string>(),
+  ctl: { account: 1 as number | null },
+}));
 vi.mock("./storage", () => ({
-  getAccountRaw: (k: string) => store.get(k) ?? null,
+  getAccountRaw: (k: string) => (ctl.account == null ? null : (store.get(k) ?? null)),
   setAccountRaw: (k: string, v: string) => {
-    store.set(k, v);
+    if (ctl.account != null) store.set(k, v);
   },
 }));
-// A stable account id keeps useAccountRehydrate a no-op here (no null→id transition); the re-hydrate
-// path itself is covered in useAccountRehydrate.dom.test.ts + useSortState.dom.test.ts.
-vi.mock("./activeAccount", () => ({ useActiveAccountId: () => 1 }));
+vi.mock("./activeAccount", () => ({ useActiveAccountId: () => ctl.account }));
 
 import { useFilterState } from "./useFilterState";
 
 describe("useFilterState", () => {
-  beforeEach(() => store.clear());
+  beforeEach(() => {
+    store.clear();
+    ctl.account = 1;
+  });
 
   it("hydrates to an empty map when storage is empty", () => {
     const { result } = renderHook(() => useFilterState("k"));
@@ -71,4 +77,42 @@ describe("useFilterState", () => {
     // A seed that returns null falls back to an empty map.
     expect(renderHook(() => useFilterState("k3", () => null)).result.current[0]).toEqual({});
   });
+
+  // The account-pin re-hydrate (via useAccountRehydrate) integrated into useFilterState.
+  it("re-hydrates the stored map once the account is pinned after mount", () => {
+    ctl.account = null; // fresh tab: no account at mount
+    store.set("k", JSON.stringify({ sync: "fully", tags: ["7"] }));
+    const { result, rerender } = renderHook(() => useFilterState("k"));
+    expect(result.current[0]).toEqual({}); // mount read with no account → empty
+    act(() => {
+      ctl.account = 1; // <App> pins the account
+      rerender();
+    });
+    expect(result.current[0]).toEqual({ sync: "fully", tags: ["7"] });
+  });
+
+  it("applies the migrate seed on the account-pin re-hydrate when the key is still empty", () => {
+    ctl.account = null;
+    // Mirrors migrateSyncChip: reads an account-scoped legacy key, so it yields null until pinned.
+    const migrate = () => (ctl.account == null ? null : { sync: "needsFull" });
+    const { result, rerender } = renderHook(() => useFilterState("k", migrate));
+    expect(result.current[0]).toEqual({}); // migrate can't read the legacy key with no account yet
+    act(() => {
+      ctl.account = 1;
+      rerender();
+    });
+    expect(result.current[0]).toEqual({ sync: "needsFull" }); // migrate carries the legacy value forward
+  });
+
+  it("does NOT clobber a pre-pin user edit when the account is then pinned (markDirty guard)", () => {
+    ctl.account = null;
+    store.set("k", JSON.stringify({ sync: "fully" })); // the stored value
+    const { result, rerender } = renderHook(() => useFilterState("k"));
+    act(() => result.current[1]({ sync: "needsFull" })); // user edits before the pin
+    act(() => {
+      ctl.account = 1;
+      rerender();
+    });
+    expect(result.current[0]).toEqual({ sync: "needsFull" }); // the edit survives, not "fully"
+  });
 });