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/playlistsInit.ts

@@ -1,14 +1,14 @@
 import { getAccountRaw, LS } from "../lib/storage";
+import { parsePositiveIntId } from "../lib/parseId";
 
 // The Playlists module's selected-id seed, split out of PlaylistsProvider so its ?pl guard is
 // unit-testable and the provider stays react-refresh-clean (component-only exports).
 export function readSelectedId(): number | null {
-  // A ?pl bookmark wins over the stored selection so a pasted link opens that playlist. Accept only a
-  // positive integer id — a bare "?pl=" gives Number("")===0 (finite!) and "?pl=1.5" a fractional id,
-  // both of which would select a non-existent playlist.
-  const fromUrl = new URLSearchParams(window.location.search).get("pl");
-  const n = fromUrl ? Number(fromUrl) : NaN;
-  if (Number.isInteger(n) && n > 0) return n;
-  const s = getAccountRaw(LS.playlist);
-  return s ? Number(s) : null;
+  // A ?pl bookmark wins over the stored selection so a pasted link opens that playlist. BOTH must be a
+  // positive-integer id (a bare "?pl=" gives Number("")===0, "?pl=1.5" a fractional id) — either would
+  // select a non-existent playlist, so the stored fallback is guarded exactly like the URL.
+  return (
+    parsePositiveIntId(new URLSearchParams(window.location.search).get("pl")) ??
+    parsePositiveIntId(getAccountRaw(LS.playlist))
+  );
 }