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/plexCards.tsx

@@ -50,6 +50,7 @@ export function PlexPosterCard({
   const { t } = useTranslation();
   const inProgress = (card.position_seconds ?? 0) > 0 && card.status !== "watched";
   const isPlayable = card.type !== "show"; // movies/episodes play; shows open the drill-down
+  const watchedLabel = card.status === "watched" ? t("plex.markUnwatched") : t("plex.markWatched");
   const pct =
     inProgress && card.duration_seconds
       ? Math.min(100, Math.round(((card.position_seconds ?? 0) / card.duration_seconds) * 100))
@@ -99,8 +100,8 @@ export function PlexPosterCard({
               e.stopPropagation();
               onToggleWatched(card);
             }}
-            title={card.status === "watched" ? t("plex.markUnwatched") : t("plex.markWatched")}
-            aria-label={card.status === "watched" ? t("plex.markUnwatched") : t("plex.markWatched")}
+            title={watchedLabel}
+            aria-label={watchedLabel}
             className="absolute top-1.5 right-1.5 p-1 rounded-full bg-black/60 reveal hover:bg-black/80 transition"
           >
             <WatchedIcon watched={card.status === "watched"} />
@@ -114,6 +115,7 @@ export function PlexPosterCard({
               onInfo();
             }}
             title={t("plex.info.title")}
+            aria-label={t("plex.info.title")}
             className="absolute bottom-1.5 left-1.5 p-1 rounded-full bg-black/60 reveal hover:bg-black/80 transition"
           >
             <Info className="w-4 h-4 text-white" />
@@ -192,6 +194,9 @@ export function SeasonCard({
 }) {
   const { t } = useTranslation();
   const watched = se.status === "watched";
+  const watchedLabel = watched
+    ? t("plex.series.markSeasonUnwatched")
+    : t("plex.series.markSeasonWatched");
   return (
     <div className="group text-left">
       <div
@@ -225,9 +230,8 @@ export function SeasonCard({
               e.stopPropagation();
               onToggleWatched();
             }}
-            title={
-              watched ? t("plex.series.markSeasonUnwatched") : t("plex.series.markSeasonWatched")
-            }
+            title={watchedLabel}
+            aria-label={watchedLabel}
             className="absolute top-1.5 right-1.5 p-1 rounded-full bg-black/60 reveal hover:bg-black/80 transition"
           >
             <WatchedIcon watched={watched} />
@@ -241,6 +245,7 @@ export function SeasonCard({
               onAdd();
             }}
             title={t("plex.playlist.addSeason")}
+            aria-label={t("plex.playlist.addSeason")}
             className="absolute bottom-1.5 right-1.5 p-1 rounded-full bg-black/60 reveal hover:bg-black/80 transition"
           >
             <ListPlus className="w-4 h-4 text-white" />
@@ -404,6 +409,7 @@ export function EpisodeCard({
 }) {
   const { t } = useTranslation();
   const inProgress = (ep.position_seconds ?? 0) > 0 && ep.status !== "watched";
+  const watchedLabel = ep.status === "watched" ? t("plex.markUnwatched") : t("plex.markWatched");
   const pct =
     inProgress && ep.duration_seconds
       ? Math.min(100, Math.round(((ep.position_seconds ?? 0) / ep.duration_seconds) * 100))
@@ -438,8 +444,8 @@ export function EpisodeCard({
               e.stopPropagation();
               onToggleWatched();
             }}
-            title={ep.status === "watched" ? t("plex.markUnwatched") : t("plex.markWatched")}
-            aria-label={ep.status === "watched" ? t("plex.markUnwatched") : t("plex.markWatched")}
+            title={watchedLabel}
+            aria-label={watchedLabel}
             className="absolute top-1.5 right-1.5 p-1 rounded-full bg-black/60 reveal hover:bg-black/80 transition"
           >
             <WatchedIcon watched={ep.status === "watched"} />