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.

test/state/season_refresh_test.dart

@@ -51,9 +51,9 @@ void main() {
     expect(repo.calls, 1);
   });
 
-  test('activeSeason picks the newest by id', () {
+  test('activeSeason starts at Season 1 (first by id)', () {
     final p1 = _pack('season_001');
     final p2 = _pack('season_002');
-    expect(activeSeason([p1, p2]).seasonId, 'season_002');
+    expect(activeSeason([p1, p2]).seasonId, 'season_001');
   });
 }