a11y(e4): aria-label on the icon-only Plex watched-toggle buttons

The hover watched-toggle on Plex movie/episode cards is icon-only (WatchedIcon,
no text), so it carried a `title` but no accessible name for screen readers —
and the empty-Circle unwatched glyph could read like a radio. Mirror the title
into `aria-label` (the Modal close-button idiom) so the control announces its
action. The PlexInfo watched buttons already have visible text labels, so they
need nothing.

FE gate green (tsc/eslint/prettier/knip, 309 vitest).

frontend/src/components/plexCards.tsx

@@ -100,6 +100,7 @@ export function PlexPosterCard({
               onToggleWatched(card);
             }}
             title={card.status === "watched" ? t("plex.markUnwatched") : t("plex.markWatched")}
+            aria-label={card.status === "watched" ? t("plex.markUnwatched") : t("plex.markWatched")}
             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"} />
@@ -438,6 +439,7 @@ export function EpisodeCard({
               onToggleWatched();
             }}
             title={ep.status === "watched" ? t("plex.markUnwatched") : t("plex.markWatched")}
+            aria-label={ep.status === "watched" ? t("plex.markUnwatched") : t("plex.markWatched")}
             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"} />