feat(series): land on the current episode, not season 1 (UR-062, UR-063, UR-064)

Opening a series dumped the viewer at the top of season 1, and its Play
button played nothing at all: it resolved `$libraryItems[0]` — the first
*season* by SortName — and navigated to `/player/<seasonId>`, which the
player route bounced straight back to `/library/<seasonId>`.

The backend could already answer "where is this viewer in this show":
`repository_get_next_up_episodes` has accepted a `series_id` since it was
written and no caller had ever passed one.

Backend (DR-101, DR-106)
- `repository/series_progress.rs`: `pick_current_episode` — in progress,
  else Next Up, else first unwatched, else the premiere. The third rung is
  the offline path, where Next Up is always empty. `sort_series_order` puts
  specials (season 0) after the numbered seasons.
- `repository_get_series_episodes` takes over the season fan-out and the
  flat-series fallback, which were domain knowledge living in the frontend.
- `clear_watch_history` maps to Jellyfin's mark-unplayed (recursive over a
  container, also zeroes resume). Offline it refuses rather than diverging
  state the next sync would undo.

Frontend (DR-102, DR-103, DR-104, DR-107)
- Seasons collapse; only the current one is expanded, and the current
  episode is badged and scrolled into view.
- Hero button reads `Resume S2E4` / `Play S1E1` and opens that episode's
  focus view, where Play commits (ux-flows §5B.5).
- Seasons are no longer a destination: `/library/<seasonId>` redirects to
  `/library/<seriesId>#season-N`, and every inbound link follows.
- The "More Episodes" strip spans the whole series, so a season finale
  offers the next premiere instead of dead-ending (§5B.2).
- Clear-history buttons on the series hero and each season header.

Routes (DR-105)
- `/library/tv` and `/library/movies` absorb their all-titles and genres
  pages as `?view=` tabs; the four legacy routes redirect. 6 video routes
  become 2, and `/library/shows/genres` stops being the odd one out.

Logic extracted to `seriesNavigation.ts`, `episodeStrip.ts` and
`libraryView.ts` so it is unit-tested rather than buried in components.
Spec: docs/specs/series-current-episode-navigation.md

src/lib/api/repository-client.ts

@@ -137,6 +137,37 @@ export class RepositoryClient {
     return commands.repositoryGetNextUpEpisodes(this.ensureHandle(), seriesId ?? null, limit ?? null);
   }
 
+  /**
+   * Every episode of a series, across all seasons, already in series order.
+   * The backend owns the season fan-out and the flat-series fallback.
+   *
+   * TRACES: UR-062 | DR-101
+   */
+  async getSeriesEpisodes(seriesId: string): Promise<MediaItem[]> {
+    return commands.repositoryGetSeriesEpisodes(this.ensureHandle(), seriesId);
+  }
+
+  /**
+   * The episode the viewer should land on when opening this series. `null` only
+   * when the series has no episodes.
+   *
+   * TRACES: UR-062 | DR-101
+   */
+  async getSeriesCurrentEpisode(seriesId: string): Promise<MediaItem | null> {
+    return commands.repositoryGetSeriesCurrentEpisode(this.ensureHandle(), seriesId);
+  }
+
+  /**
+   * Erase watch history for an item. On a series or season the server applies
+   * it to everything inside, so the container returns to "never watched".
+   * Requires the server — this fails offline rather than diverging local state.
+   *
+   * TRACES: UR-064 | DR-106
+   */
+  async clearWatchHistory(itemId: string): Promise<void> {
+    await commands.repositoryClearWatchHistory(this.ensureHandle(), itemId);
+  }
+
   async getRecentlyPlayedAudio(limit?: number): Promise<MediaItem[]> {
     return commands.repositoryGetRecentlyPlayedAudio(this.ensureHandle(), limit ?? null);
   }