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/ui/screens/settings_screen.dart

@@ -76,6 +76,16 @@ class SettingsScreen extends ConsumerWidget {
                 title: Text(l10n.restorePurchases),
                 onTap: () => iap.restorePurchases(),
               ),
+              const SizedBox(height: 24),
+              Center(
+                child: Text(
+                  'Block Seasons · v1.0.0 (build 3)',
+                  style: TextStyle(
+                    color: Colors.white.withValues(alpha: 0.4),
+                    fontSize: 12,
+                  ),
+                ),
+              ),
             ],
           ),
         ),