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

LeaderboardScreen called loadAll() which uses Future.wait across 4 APIs.
If getDashboard() fails (empty user_id before auth), the entire batch
throws and leaderboard stays []. Switching districts worked because
setDistrict() calls getLeaderboard() directly.

- Add loadLeaderboard() to GamificationProvider — calls only getLeaderboard(),
  independent of dashboard/shop/achievements
- Add isLeaderboardLoading field with correct lifecycle in setDistrict/setTimePeriod
- LeaderboardScreen.initState now calls loadLeaderboard() instead of loadAll()

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

lib/features/gamification/providers/gamification_provider.dart

@@ -22,6 +22,7 @@ class GamificationProvider extends ChangeNotifier {
   String leaderboardTimePeriod = 'all_time'; // 'all_time' | 'this_month'
 
   bool isLoading = false;
+  bool isLeaderboardLoading = false;
   String? error;
 
   // TTL guard — prevents redundant API calls from multiple screens
@@ -80,12 +81,36 @@ class GamificationProvider extends ChangeNotifier {
     }
   }
 
+  // ---------------------------------------------------------------------------
+  // Load only leaderboard data — safe to call independently of loadAll().
+  // Used by LeaderboardScreen so a dashboard/shop failure doesn't blank the list.
+  // ---------------------------------------------------------------------------
+  Future<void> loadLeaderboard() async {
+    if (isLeaderboardLoading) return;
+    isLeaderboardLoading = true;
+    notifyListeners();
+    try {
+      final response = await _service.getLeaderboard(
+        district: leaderboardDistrict,
+        timePeriod: leaderboardTimePeriod,
+      );
+      leaderboard = response.entries;
+      currentUserStats = response.currentUser;
+      totalParticipants = response.totalParticipants;
+    } catch (e) {
+      error = userFriendlyError(e);
+    }
+    isLeaderboardLoading = false;
+    notifyListeners();
+  }
+
   // ---------------------------------------------------------------------------
   // Change district filter
   // ---------------------------------------------------------------------------
   Future<void> setDistrict(String district) async {
     if (leaderboardDistrict == district) return;
     leaderboardDistrict = district;
+    isLeaderboardLoading = true;
     notifyListeners();
     try {
       final response = await _service.getLeaderboard(district: district, timePeriod: leaderboardTimePeriod);
@@ -95,6 +120,7 @@ class GamificationProvider extends ChangeNotifier {
     } catch (e) {
       error = userFriendlyError(e);
     }
+    isLeaderboardLoading = false;
     notifyListeners();
   }
 
@@ -104,6 +130,7 @@ class GamificationProvider extends ChangeNotifier {
   Future<void> setTimePeriod(String period) async {
     if (leaderboardTimePeriod == period) return;
     leaderboardTimePeriod = period;
+    isLeaderboardLoading = true;
     notifyListeners();
     try {
       final response = await _service.getLeaderboard(district: leaderboardDistrict, timePeriod: period);
@@ -113,6 +140,7 @@ class GamificationProvider extends ChangeNotifier {
     } catch (e) {
       error = userFriendlyError(e);
     }
+    isLeaderboardLoading = false;
     notifyListeners();
   }