feat(ui): season tabs replace two-level seasons navigation

SeasonTabsComponent renders a pill row (dropdown beyond 6 seasons), an
optional season description and a back-to-playing chip. The season
container auto-selects a season (inline-playing episode's season →
most recent in-progress → first), emits seasonSelected for
auto-selections so lazy-load/enrichment hooks keep firing, highlights
the inline-playing episode, and loses the seasons grid + 'Back to
seasons' button. Download/progress helpers move to pure utils; season
strings are now translatable.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

apps/web/src/assets/i18n/en.json

@@ -922,6 +922,15 @@
         "CLOSE_PLAYER": "Close player",
         "BACK_TO_BROWSE": "Back to details",
         "ABOUT": "About",
+        "SEASONS_AND_EPISODES": "Seasons and Episodes",
+        "SEASON_TAB": "Season {{number}}",
+        "BACK_TO_PLAYING_EPISODE": "Back to playing episode",
+        "GRID_VIEW": "Grid view",
+        "LIST_VIEW": "List view",
+        "NO_EPISODES_AVAILABLE": "No episodes available",
+        "NO_EPISODES_AVAILABLE_MESSAGE": "This series is listed by your provider, but no seasons or episodes are available to play.",
+        "SEASON_EMPTY": "This season is empty",
+        "SEASON_EMPTY_MESSAGE": "This season is listed by your provider, but no playable episodes are available.",
         "SELECT_CATEGORY": "Select a category",
         "PAGE": "Page",
         "CONTENT_NOT_AVAILABLE": "This content is not available on this server",