Resume tracking: per-episode positions, player-agnostic fallback, series continue point

- Fix merged playback-state dropping season/episode, which broke series
  resume restore entirely.
- Track watch progress per episode (episodes map keyed S..E.., with done
  markers on completion) while keeping one continue point per series as
  the last watched episode, used for spoiler protection and season
  selection. MPC-BE tracker resumes from each episode's own position.
- Assumed-playback fallback for any player and server-only mode: a launch
  starts a session and the guessed position (base + elapsed, capped at
  runtime) is written when frontend activity resumes; the real MPC-BE
  tracker overrides guesses via timestamps. Frontend reports input
  activity (throttled) via POST /api/activity.
- Ignore watches under 5 minutes in both trackers.
- Episode tiles show a small quadrant-circle watch indicator; near end
  counts as fully watched, no data shows nothing.
- Fix episode audio ownership: single audioOwnerKey shared by mouse hover
  and keyboard/gamepad focus, no longer clobbered by playback sync;
  idle fade can be re-armed by continued activity.

frontend/src/types.ts

@@ -109,6 +109,13 @@ export interface Series {
   seasons: Season[]
 }
 
+/** A series' single continue point (last watched position). */
+export interface SeriesResumePoint {
+  seasonNumber: number
+  episodeNumber: number
+  positionSeconds: number
+}
+
 export interface MovieUi extends Movie {
   id: string
   root_id: string | null