feat(anime): filter episodes and mark them watched by hand

- Add a filter box above the episode list for a number, range, or name substring, with a "N of M" counter
- Read watch marks from the immersion tracker stats and show them per episode, with a watched count in the header, refreshed on window focus
- Add a right-click menu to mark one episode or a whole catch-up span (this and everything below) watched/unwatched
- Add setWatched/getWatchState IPC plumbing so a manual mark creates the stats row for an episode that was never played

src/animeui/episode-marks.ts

@@ -0,0 +1,21 @@
+/**
+ * Which episodes a manual watch mark applies to.
+ *
+ * Sources list newest first, so "everything below" the row you right-clicked is
+ * the back catalogue: the natural shape of "I have watched up to here". The
+ * span is taken from the full list rather than from what the filter leaves,
+ * because a filter narrows what you are looking at, not what you have watched.
+ */
+
+export type MarkScope = 'one' | 'below';
+
+export function episodesInScope<T>(episodes: T[], index: number, scope: MarkScope): T[] {
+  if (index < 0 || index >= episodes.length) return [];
+  return scope === 'one' ? [episodes[index]!] : episodes.slice(index);
+}
+
+/** "3 episodes" / "1 episode", for the status line after a bulk mark. */
+export function describeMarkCount(count: number, watched: boolean): string {
+  const noun = count === 1 ? 'episode' : 'episodes';
+  return watched ? `Marked ${count} ${noun} watched` : `Cleared the watch mark on ${count} ${noun}`;
+}