fix(e4): address code-review findings (a11y sweep + dedup guards)

Resolves the 5 /code-review high findings on this branch:
- a11y sweep: extend aria-label to the remaining icon-only Plex card buttons the
  first pass missed — PlexPosterCard Info, SeasonCard watched-toggle + add-season
  (EpisodeCard already had them). "Fix a principle everywhere, in one sweep."
- new shared lib/parseId.ts `parsePositiveIntId`, used by BOTH readSelectedId
  (now guarding the stored ?pl fallback the same as the URL — was unguarded) and
  plexSubUrl's ?v=playlist decode; removes the hand-duplicated guard.
- PAGE_KEY exported from navigationInit and imported by NavigationProvider (was
  defined in both).
- hoist a `watchedLabel` const per card so title + aria-label can't drift.

+8 tests (parseId unit + guarded-stored ?pl); full FE gate green (tsc/eslint/
prettier/knip, 317 vitest).

frontend/src/components/NavigationProvider.tsx

@@ -11,10 +11,10 @@ import {
 import { useTranslation } from "react-i18next";
 import { urlWithParams, type Page } from "../lib/urlState";
 import { pageTitleKey } from "../lib/pageMeta";
-import { LS, setAccountRaw } from "../lib/storage";
+import { setAccountRaw } from "../lib/storage";
 import { useAccountRehydrate } from "../lib/useAccountRehydrate";
 import { setNavigator } from "../lib/nav";
-import { channelViewFrom, loadInitialPage, type ChannelView } from "./navigationInit";
+import { channelViewFrom, loadInitialPage, PAGE_KEY, type ChannelView } from "./navigationInit";
 
 // Owns the app's navigation + browser-history state, lifted out of the App god component so pages
 // consume it via a hook instead of prop-drilling `page`/`setPage`/`onOpenChannel` through the tree.
@@ -27,8 +27,6 @@ import { channelViewFrom, loadInitialPage, type ChannelView } from "./navigation
 // re-render on a page/channel switch — preserving the memo that keeps feed appends from re-rendering
 // every existing card.
 
-const PAGE_KEY = LS.page;
-
 export interface NavigationState {
   page: Page;
   /** The open channel page (null = none). Overlays `page`; rides in history.state._chan. */