feat: start at Season 1, add menu/season_001 BGM tracks, settings build tag

- activeSeason now returns the first season (Season 1 'First Bloom') so a new
  player starts at spring instead of the newest season.
- Bundle the owner-picked CC0 tracks menu.mp3 + season_001.mp3 (BGM now audible).
- Settings footer shows 'v1.0.0 (build 3)' so test builds are identifiable.
180 tests green, analyze clean.

lib/state/providers.dart

@@ -76,9 +76,12 @@ final seasonRefreshProvider = FutureProvider<bool>(
   (ref) => ref.read(contentRepositoryProvider).refresh(),
 );
 
-/// The season players land in by default: the newest available.
-/// (availableSeasons is sorted by seasonId ascending.)
-SeasonPack activeSeason(List<SeasonPack> seasons) => seasons.last;
+/// The season players land in by default: Season 1, the start of the journey,
+/// so a new player experiences the content in order rather than being dropped
+/// into the newest season. (availableSeasons is sorted by seasonId ascending,
+/// so `.first` is Season 1.) A season selector to reach later seasons is a
+/// planned follow-up.
+SeasonPack activeSeason(List<SeasonPack> seasons) => seasons.first;
 
 final streakProvider = NotifierProvider<StreakNotifier, StreakState>(
   StreakNotifier.new,