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/api/core.ts

@@ -5,7 +5,7 @@
 import { notify, remove } from "../notifications";
 import i18n from "../../i18n";
 import { reportError } from "../errorDialog";
-import { activeAccountId, ACTIVE_ACCOUNT_KEY, notifyActiveAccountChange } from "../storage";
+import { activeAccountId, ACTIVE_ACCOUNT_KEY, notifyActiveAccountChange } from "../activeAccount";
 
 export class HttpError extends Error {
   status: number;