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/lib/useAccountRehydrate.ts

@@ -1,5 +1,5 @@
 import { useCallback, useEffect, useRef } from "react";
-import { useActiveAccountId } from "./storage";
+import { useActiveAccountId } from "./activeAccount";
 
 // Re-read per-account persisted state when this tab's active account becomes known (or changes). The
 // provider tree above <App> mounts before the account is pinned, so its useState initializers seed from
@@ -9,6 +9,11 @@ import { useActiveAccountId } from "./storage";
 //
 // Keyed on the account id ONLY — the user's own writes go through setAccountRaw, which does not change
 // the account id, so this effect can't loop on them.
+//
+// This is the localStorage-only counterpart of the `useEffect([me?.id])` server-prefs adopt pattern in
+// FeedFiltersProvider / FeedViewProvider / PrefsProvider (and App's shell prefs): those re-load from the
+// SERVER when the account query resolves; this re-loads from localStorage when the account is pinned.
+// A provider needs one or the other — not both — to survive a fresh tab's first paint.
 export function useAccountRehydrate(rehydrate: () => void): () => void {
   const accountId = useActiveAccountId();
   const dirty = useRef(false);
@@ -22,6 +27,10 @@ export function useAccountRehydrate(rehydrate: () => void): () => void {
     if (accountId === prev.current) return; // mount, and renders with no real transition
     prev.current = accountId;
     if (!dirty.current) fn.current();
+    // Reset per account: `dirty` means "the user edited under THIS account". Today a real account
+    // switch full-reloads (fresh instance), but clearing it here keeps the guard correct even if a
+    // future switch stops reloading — the next account starts clean rather than inheriting dirtiness.
+    dirty.current = false;
   }, [accountId]);
 
   return useCallback(() => {