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/bindings.ts

@@ -1267,6 +1267,46 @@ async repositoryGetResumeItems(handle: string, parentId: string | null, limit: n
 async repositoryGetNextUpEpisodes(handle: string, seriesId: string | null, limit: number | null) : Promise<MediaItem[]> {
     return await TAURI_INVOKE("repository_get_next_up_episodes", { handle, seriesId, limit });
 },
+/**
+ * Every episode of a series, across all seasons, in series order.
+ * 
+ * Jellyfin hangs episodes off season folders — except for "flat" series whose
+ * children are episodes directly. Both shapes are provider vocabulary, so the
+ * fan-out and its fallback live in Rust rather than being reimplemented in the
+ * frontend (which is what it used to do).
+ * 
+ * TRACES: UR-062 | DR-101
+ */
+async repositoryGetSeriesEpisodes(handle: string, seriesId: string) : Promise<MediaItem[]> {
+    return await TAURI_INVOKE("repository_get_series_episodes", { handle, seriesId });
+},
+/**
+ * The episode a viewer should land on when they open a series.
+ * 
+ * "Current" is domain policy, not layout: an episode in progress, else the
+ * server's Next Up for the series, else the first unwatched episode, else the
+ * first. The third rung is what makes this work offline, where Next Up is
+ * always empty. Returns `None` only when the series has no episodes at all.
+ * 
+ * TRACES: UR-062 | DR-101
+ */
+async repositoryGetSeriesCurrentEpisode(handle: string, seriesId: string) : Promise<MediaItem | null> {
+    return await TAURI_INVOKE("repository_get_series_current_episode", { handle, seriesId });
+},
+/**
+ * Erase the viewer's watch history for an item.
+ * 
+ * Clears the played flag and the resume position; on a series or season the
+ * server applies it to everything inside. A series cleared this way is "never
+ * watched" again, so `repository_get_series_current_episode` returns its
+ * premiere. Requires the server — offline this fails rather than diverging
+ * local state the next sync would overwrite.
+ * 
+ * TRACES: UR-064 | DR-106
+ */
+async repositoryClearWatchHistory(handle: string, itemId: string) : Promise<null> {
+    return await TAURI_INVOKE("repository_clear_watch_history", { handle, itemId });
+},
 /**
  * Get recently played audio
  */