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/components/library/episodeStrip.ts

@@ -1,12 +1,20 @@
 // Pure logic for the "More Episodes" strip in EpisodeFocusView.
 //
 // Extracted from the component so it can be unit-tested: the strip must never
-// collapse to just the current episode while real siblings exist, and it must
-// not mistake number-less episodes for the current one.
+// collapse to just the current episode while real siblings exist, must not
+// mistake number-less episodes for the current one, and must run past a season
+// boundary rather than dead-ending at the end of a season (ux-flows §5B.2).
 //
-// TRACES: UR-048 | DR-062
+// TRACES: UR-048, UR-062 | DR-062, DR-104
 import type { MediaItem } from "$lib/api/types";
 
+/** Episodes shown before / after the current one in the strip window. */
+const BEFORE = 3;
+const AFTER = 6;
+
+/** Jellyfin puts specials in season 0; they air outside the numbered run. */
+const SPECIALS_SEASON = 0;
+
 /**
  * Does `ep` refer to the same episode as `current`?
  *
@@ -29,33 +37,74 @@ export function isCurrentEpisode(ep: MediaItem, current: MediaItem): boolean {
   );
 }
 
+/**
+ * Sort key for a season: specials (season 0) come *after* every numbered
+ * season, matching how a viewer works through a show — S1, S2, …, then the
+ * extras — rather than opening on a special because 0 < 1.
+ */
+function seasonRank(seasonNumber: number | null | undefined): number {
+  return seasonNumber === SPECIALS_SEASON ? Number.MAX_SAFE_INTEGER : seasonNumber!;
+}
+
+/**
+ * Broadcast order across a whole series: season ascending, then episode.
+ *
+ * When *either* side's season is unknown there is no season axis to compare on,
+ * so it falls through to episode number. That makes the comparator technically
+ * non-transitive across such a mix, which is safe here because only the
+ * directly-fetched `current` episode can lack a season and it is never part of
+ * the array being sorted — it is only positioned against it (see
+ * `adjacentEpisodes`).
+ */
+export function compareSeriesOrder(a: MediaItem, b: MediaItem): number {
+  if (a.parentIndexNumber != null && b.parentIndexNumber != null) {
+    const bySeason = seasonRank(a.parentIndexNumber) - seasonRank(b.parentIndexNumber);
+    if (bySeason !== 0) return bySeason;
+  }
+  return (a.indexNumber ?? 0) - (b.indexNumber ?? 0);
+}
+
 /**
  * The window of episodes shown under the hero: up to 3 before and 6 after the
- * current episode. Degrades gracefully:
- *  - prefers the current season, falling back to the full list when the season
- *    is unknown (e.g. the episode was fetched directly on an API-ID mismatch);
- *  - splices the current episode into the pool at its numeric position when it
- *    isn't present, so it still anchors the window;
+ * current episode, in series order across *all* seasons.
+ *
+ * Crossing a season boundary is the point (ux-flows §5B.2): finishing a season
+ * finale should offer the next season's premiere, not an empty strip. Degrades
+ * gracefully:
+ *  - splices the current episode into the pool at its ordered position when it
+ *    isn't present (an API id mismatch on a directly-fetched episode), so it
+ *    still anchors the window;
  *  - returns just `[current]` only when there genuinely are no other episodes.
  */
 export function adjacentEpisodes(current: MediaItem, allEpisodes: MediaItem[]): MediaItem[] {
-  const seasonMatches = allEpisodes.filter(
-    (e) => current.parentIndexNumber != null && e.parentIndexNumber === current.parentIndexNumber
-  );
-  const pool = (seasonMatches.length > 0 ? seasonMatches : allEpisodes)
-    .slice()
-    .sort((a, b) => (a.indexNumber ?? 0) - (b.indexNumber ?? 0));
+  const pool = allEpisodes.slice().sort(compareSeriesOrder);
 
   let idx = pool.findIndex((e) => isCurrentEpisode(e, current));
 
   if (idx === -1) {
-    const epNum = current.indexNumber ?? 0;
-    const insertAt = pool.findIndex((e) => (e.indexNumber ?? 0) > epNum);
+    const insertAt = pool.findIndex((e) => compareSeriesOrder(e, current) > 0);
     idx = insertAt === -1 ? pool.length : insertAt;
     pool.splice(idx, 0, current);
   }
 
-  const start = Math.max(0, idx - 3);
-  const end = Math.min(pool.length, idx + 7);
+  const start = Math.max(0, idx - BEFORE);
+  const end = Math.min(pool.length, idx + AFTER + 1);
   return pool.slice(start, end);
 }
+
+/**
+ * Label for a strip card, relative to the episode in focus.
+ *
+ * Within the current season a bare number reads cleanly ("6."). Once the window
+ * crosses into another season that number is ambiguous, so the card names the
+ * season too ("S3E1") — otherwise the premiere after a finale just reads "1."
+ */
+export function stripCardLabel(ep: MediaItem, current: MediaItem): string {
+  const crossesSeason =
+    ep.parentIndexNumber != null &&
+    current.parentIndexNumber != null &&
+    ep.parentIndexNumber !== current.parentIndexNumber;
+
+  if (crossesSeason) return `S${ep.parentIndexNumber}E${ep.indexNumber ?? 0}`;
+  return `${ep.indexNumber ?? 0}.`;
+}