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/shared/ipc/contracts.ts

@@ -132,6 +132,8 @@ export const IPC_CHANNELS = {
     animeBrowserGetPopular: 'anime-browser:get-popular',
     animeBrowserGetDetails: 'anime-browser:get-details',
     animeBrowserGetEpisodes: 'anime-browser:get-episodes',
+    animeBrowserGetWatchState: 'anime-browser:get-watch-state',
+    animeBrowserSetWatched: 'anime-browser:set-watched',
     animeBrowserPlayEpisode: 'anime-browser:play-episode',
     animeBrowserGetPreferences: 'anime-browser:get-preferences',
     animeBrowserSetPreference: 'anime-browser:set-preference',