refactor(account): address account-pin re-hydrate review — extract module, harden guards

Fixes the 5 findings from /code-review high on the account-pin work:
- Extract the reactive-account primitives (ACTIVE_ACCOUNT_KEY, activeAccountId,
  notifyActiveAccountChange, useActiveAccountId) into a new dependency-free
  lib/activeAccount.ts, breaking the storage.ts <-> useAccountRehydrate.ts import cycle.
  storage.ts / api/core.ts / useAccountRehydrate.ts now import from it.
- NavigationProvider: the page re-hydrate now PRESERVES existing history.state markers
  (spread, not overwrite), and openChannel/enterYtSearch markDirty so a sub-view opened
  before the account pins isn't clobbered by the page override.
- useAccountRehydrate: reset the dirty flag after each account transition, so a future
  non-reloading account switch can't skip the re-hydrate and leak the prior account's state.
- Doc-link useAccountRehydrate to the pre-existing useEffect([me?.id]) server-adopt pattern.
- Tests: model a mutable pinned account and add useSortState integration tests for the
  null->id re-hydrate + the markDirty no-clobber guard (vitest 115->117).

gate-green: vite build + vitest 117 + eslint-0 + knip + prettier. Self-E2E'd on a fresh
tab: still lands on Plex with Movies/In-progress/Year restored, zero console errors.

frontend/src/components/NavigationProvider.tsx

@@ -114,7 +114,10 @@ export function NavigationProvider({ children }: { children: ReactNode }) {
     if (stored === pageRef.current) return;
     setPageState(stored);
     setAccountRaw(PAGE_KEY, stored); // no-op re-write, keeps storage authoritative
-    window.history.replaceState({ sfPage: stored }, "");
+    // 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.)
+    window.history.replaceState({ ...(window.history.state || {}), sfPage: stored }, "");
   });
 
   const setPage = useCallback(
@@ -134,27 +137,35 @@ export function NavigationProvider({ children }: { children: ReactNode }) {
     [markDirty],
   );
 
-  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 }, "");
-  }, []);
+  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
+      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) => {
-    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.
-    const { _ytScrape: _drop, ...st } = window.history.state || {};
-    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
-  }, []);
+  const enterYtSearch = useCallback(
+    (q: string) => {
+      markDirty(); // a sub-view navigation — see openChannel
+      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.
+      const { _ytScrape: _drop, ...st } = window.history.state || {};
+      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.