feat(anime): queue episodes to play next across anime

- Episode rows gain Play/Queue actions (and matching context-menu items); queued rows show their place in line, with a queue count and Clear queue in the episode header
- Queue lives in the main process (`anime-browser-queue.ts`) so it survives the browser window closing and advances on mpv's end-file even when nobody is watching; streams resolve at play time so a signed URL cannot expire while queued
- Holds mpv's keep-open off while the queue waits and restores it once empty; queueing with nothing playing just plays immediately
- Adds anime-browser-queue and episode-queue unit tests, IPC channels/contracts, and doc updates

src/shared/ipc/contracts.ts

@@ -135,6 +135,11 @@ export const IPC_CHANNELS = {
     animeBrowserGetWatchState: 'anime-browser:get-watch-state',
     animeBrowserSetWatched: 'anime-browser:set-watched',
     animeBrowserPlayEpisode: 'anime-browser:play-episode',
+    animeBrowserQueueEpisode: 'anime-browser:queue-episode',
+    animeBrowserDequeueEpisode: 'anime-browser:dequeue-episode',
+    animeBrowserClearQueue: 'anime-browser:clear-queue',
+    animeBrowserGetQueue: 'anime-browser:get-queue',
+    animeBrowserIsPlaying: 'anime-browser:is-playing',
     animeBrowserGetPreferences: 'anime-browser:get-preferences',
     animeBrowserSetPreference: 'anime-browser:set-preference',
     animeBrowserListAvailableExtensions: 'anime-browser:list-available-extensions',
@@ -179,6 +184,7 @@ export const IPC_CHANNELS = {
     syncUiStateChanged: 'sync-ui:state-changed',
     animeBrowserBridgeState: 'anime-browser:bridge-state',
     animeBrowserSearchUpdate: 'anime-browser:search-update',
+    animeBrowserQueueState: 'anime-browser:queue-state',
   },
 } as const;