fix: leaderboard empty on first open — decouple from loadAll()

- Add isLeaderboardLoading flag separate from isLoading
- Add loadLeaderboard() method that fires independently of loadAll TTL
- Remove leaderboard from loadAll() Future.wait (failures in dashboard/shop
  no longer silently zero-out leaderboard data)
- setDistrict / setTimePeriod now use isLeaderboardLoading
- contribute_screen calls loadLeaderboard() alongside loadAll() on mount

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/screens/contribute_screen.dart

@@ -104,7 +104,9 @@ class _ContributeScreenState extends State<ContributeScreen>
     super.initState();
     PostHogService.instance.screen('Contribute');
     WidgetsBinding.instance.addPostFrameCallback((_) {
-      context.read<GamificationProvider>().loadAll();
+      final p = context.read<GamificationProvider>();
+      p.loadAll();
+      p.loadLeaderboard(); // independent — always fires regardless of loadAll TTL
     });
   }
 
@@ -274,7 +276,7 @@ class _ContributeScreenState extends State<ContributeScreen>
         const SizedBox(height: 16),
 
         // Leaderboard List
-        if (provider.isLoading && leaderboard.isEmpty)
+        if (provider.isLeaderboardLoading && leaderboard.isEmpty)
           const Padding(
             padding: EdgeInsets.symmetric(vertical: 60),
             child: Center(child: CircularProgressIndicator(strokeWidth: 2)),