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

docs/ux-flows.md

@@ -688,10 +688,10 @@ A movie has no continuation set, so cast follows the hero directly.
 ### 5B.4 Series detail — section order
 
 ```
-Hero (poster, title, metadata, Play / Download)
+Hero (poster, title, metadata, Resume SxEy / Download / Clear history)
   → Crew links
   → Genre tags
-  → Seasons + episodes (per-season sections)
+  → Seasons (collapsible; only the current season expanded)
   → Cast
   → More Like This
 ```
@@ -700,6 +700,29 @@ The same principle as §5B.2: **episodes come before cast and similar shows.**
 The reason a user opens a series page is to pick an episode; discovery content
 is secondary and sits underneath.
 
+**Rules for the seasons block** *(UR-062, UR-064)*:
+
+- **The page opens where the viewer is.** The backend resolves the current
+  episode — in progress, else Next Up, else first unwatched, else the premiere —
+  and the page scrolls it into view with an `Up next` badge and a highlight ring.
+  Never season 1 by default, unless season 1 *is* where the viewer is.
+- **Seasons collapse; only the current one is expanded.** A ten-season show
+  otherwise renders hundreds of rows and buries the episode the viewer came for.
+  A collapsed season still names its episode count and watched count, so
+  progress is readable without expanding it.
+- **The hero button opens, it does not play.** It reads `Resume S2E4` /
+  `Play S1E1` — naming its target — and navigates to that episode's Focus View,
+  where Play commits. Play on a *container* is navigation (§5B.5); Play on a
+  *leaf* is the commitment.
+- **A season is never its own page.** `/library/<seasonId>` redirects to
+  `/library/<seriesId>#season-N`. Every affordance that names a season — the
+  episode breadcrumb, a season card in a grid, a Downloads drill-in — lands on
+  the series with that season in view, so the episodes of all seasons stay one
+  browsable list.
+- **Watch history is erasable** per series (hero) and per season (season
+  header). It confirms first, cannot be undone, and needs the server. Clearing a
+  whole series returns it to S1E1 by the same path a never-watched show takes.
+
 ### 5B.5 Home-card interaction — tap opens, long-press plays
 
 Cards on the Home screen carousels (Next Movie, Next Episode, Continue