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/SeasonSection.svelte

@@ -1,26 +1,56 @@
+<!-- TRACES: UR-062, UR-064 | DR-102, DR-103, DR-106, DR-107 -->
 <script lang="ts">
   import type { MediaItem } from "$lib/api/types";
   import EpisodeRow from "./EpisodeRow.svelte";
   import SeasonDownloadButton from "./SeasonDownloadButton.svelte";
+  import ClearHistoryButton from "./ClearHistoryButton.svelte";
   import CachedImage from "$lib/components/common/CachedImage.svelte";
+  import { seasonAnchorId } from "./seriesNavigation";
 
   interface Props {
     season: MediaItem;
     episodes: MediaItem[];
     focusedEpisodeId?: string;
+    /** The episode the viewer is up to — highlighted and scrolled into view. */
+    currentEpisodeId?: string;
+    /**
+     * Whether this season's episode list is open. Only the current season
+     * starts expanded, so a ten-season show does not render every episode at
+     * once. TRACES: UR-062 | DR-107
+     */
+    expanded?: boolean;
+    onToggle?: () => void;
     onEpisodeClick?: (episode: MediaItem) => void;
+    onHistoryCleared?: () => void;
   }
 
-  let { season, episodes, focusedEpisodeId, onEpisodeClick }: Props = $props();
+  let {
+    season,
+    episodes,
+    focusedEpisodeId,
+    currentEpisodeId,
+    expanded = false,
+    onToggle,
+    onEpisodeClick,
+    onHistoryCleared,
+  }: Props = $props();
+
+  const holdsCurrentEpisode = $derived(
+    currentEpisodeId != null && episodes.some((e) => e.id === currentEpisodeId)
+  );
+  const watchedCount = $derived(episodes.filter((e) => e.userData?.isPlayed).length);
 
   const episodeCount = $derived(episodes.length);
-  const seasonNumber = $derived(season.indexNumber || season.parentIndexNumber);
+  const seasonNumber = $derived(season.indexNumber ?? season.parentIndexNumber);
   const seasonName = $derived(
-    season.name || (seasonNumber ? `Season ${seasonNumber}` : "Unknown Season")
+    season.name || (seasonNumber != null ? `Season ${seasonNumber}` : "Unknown Season")
   );
+  // Seasons have no page of their own; a season link scrolls to this anchor
+  // inside the series' single continuous episode list.
+  const anchor = $derived(seasonAnchorId(seasonNumber));
 </script>
 
-<section class="space-y-4">
+<section class="space-y-4 scroll-mt-4" id={anchor}>
   <!-- Season header -->
   <div class="flex gap-4 p-4 bg-[var(--color-surface)] rounded-xl">
     <!-- Season poster -->
@@ -38,28 +68,60 @@
     <!-- Season info -->
     <div class="flex-1 min-w-0">
       <div class="flex items-start justify-between gap-4">
-        <div class="flex-1 min-w-0">
-          <h2 class="text-xl font-bold text-white">
-            {seasonName}
+        <!-- The whole title block toggles the season open/closed. -->
+        <button
+          type="button"
+          onclick={onToggle}
+          aria-expanded={expanded}
+          aria-controls="{anchor}-episodes"
+          class="flex-1 min-w-0 text-left group/season"
+        >
+          <h2 class="text-xl font-bold text-white flex items-center gap-2">
+            <svg
+              class="w-5 h-5 flex-shrink-0 text-gray-400 transition-transform duration-200
+                     group-hover/season:text-white {expanded ? 'rotate-90' : ''}"
+              fill="none"
+              stroke="currentColor"
+              stroke-width="2"
+              viewBox="0 0 24 24"
+              aria-hidden="true"
+            >
+              <path stroke-linecap="round" stroke-linejoin="round" d="M9 5l7 7-7 7" />
+            </svg>
+            <span class="truncate">{seasonName}</span>
+            {#if holdsCurrentEpisode}
+              <span
+                class="flex-shrink-0 px-2 py-0.5 rounded bg-yellow-400 text-black text-xs font-semibold"
+              >
+                Up next
+              </span>
+            {/if}
           </h2>
 
-          <div class="flex items-center gap-3 mt-1 text-sm text-gray-400">
+          <div class="flex items-center gap-3 mt-1 text-sm text-gray-400 pl-7">
             <span>{episodeCount} {episodeCount === 1 ? "Episode" : "Episodes"}</span>
+            <!-- Collapsed, this is the only progress signal the season shows. -->
+            {#if watchedCount > 0}
+              <span>•</span>
+              <span>
+                {watchedCount === episodeCount ? "Watched" : `${watchedCount} watched`}
+              </span>
+            {/if}
             {#if season.productionYear}
               <span>•</span>
               <span>{season.productionYear}</span>
             {/if}
           </div>
 
-          {#if season.overview}
-            <p class="text-gray-400 text-sm mt-3 line-clamp-3">
+          {#if season.overview && expanded}
+            <p class="text-gray-400 text-sm mt-3 line-clamp-3 pl-7">
               {season.overview}
             </p>
           {/if}
-        </div>
+        </button>
 
-        <!-- Download Season Button -->
-        <div class="flex-shrink-0">
+        <!-- Per-season actions -->
+        <div class="flex-shrink-0 flex items-center gap-2">
           <SeasonDownloadButton
             seasonId={season.id}
             seriesName={season.seriesName || ""}
@@ -68,19 +130,29 @@
             {episodeCount}
             size="sm"
           />
+          <ClearHistoryButton
+            itemId={season.id}
+            itemName={seasonName}
+            scope="season"
+            size="sm"
+            onCleared={onHistoryCleared}
+          />
         </div>
       </div>
     </div>
   </div>
 
   <!-- Episode list -->
-  <div class="space-y-1 pl-2">
-    {#each episodes as episode (episode.id)}
-      <EpisodeRow
-        {episode}
-        focused={episode.id === focusedEpisodeId}
-        onclick={() => onEpisodeClick?.(episode)}
-      />
-    {/each}
-  </div>
+  {#if expanded}
+    <div class="space-y-1 pl-2" id="{anchor}-episodes">
+      {#each episodes as episode (episode.id)}
+        <EpisodeRow
+          {episode}
+          focused={episode.id === focusedEpisodeId}
+          current={episode.id === currentEpisodeId}
+          onclick={() => onEpisodeClick?.(episode)}
+        />
+      {/each}
+    </div>
+  {/if}
 </section>