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/plexSubUrl.ts

@@ -1,4 +1,5 @@
 import type { SubviewUrl } from "../lib/history";
+import { parsePositiveIntId } from "../lib/parseId";
 
 // The Plex module's sub-view union and its address-bar mirror, extracted from PlexBrowse so the
 // encode/decode round-trip (and its guards) can be unit-tested without pulling the whole component
@@ -46,9 +47,9 @@ export const PLEX_SUB_URL: SubviewUrl<Sub> = {
         return arg ? { kind: "person", name: arg } : null;
       case "playlist": {
         // Positive-integer id only — a fractional/zero/negative ?v=playlist:<arg> would open a
-        // sub-view for a non-existent id (matches the ?pl guard in PlaylistsProvider.readSelectedId).
-        const id = arg ? Number(arg) : NaN;
-        return Number.isInteger(id) && id > 0 ? { kind: "playlist", id } : null;
+        // sub-view for a non-existent id (shares parsePositiveIntId with readSelectedId's ?pl guard).
+        const id = parsePositiveIntId(arg);
+        return id != null ? { kind: "playlist", id } : null;
       }
       case "season": {
         const [showId, seasonId] = arg.split(":");