feat: PostHog analytics wiring across all key screens

- Commit untracked posthog_service.dart (fire-and-forget HTTP client,
  EU data residency, already used by auth for identify/reset)
- screen() calls: Home, Contribute, Profile, EventDetail (with event_id)
- capture('event_tapped') on hero carousel card tap (source: hero_carousel)
- capture('book_now_tapped') in _navigateToCheckout (event_id + name)
- capture('review_submitted') in _handleSubmit (event_id + rating)
- Covers all 4 expansion items from security audit finding 8.2

lib/screens/profile_screen.dart

@@ -24,6 +24,7 @@ import '../core/app_decoration.dart';
 import '../core/constants.dart';
 import '../widgets/landscape_section_header.dart';
 import '../features/share/share_rank_card.dart';
+import '../core/analytics/posthog_service.dart';
 
 class ProfileScreen extends StatefulWidget {
   const ProfileScreen({Key? key}) : super(key: key);
@@ -99,6 +100,7 @@ class _ProfileScreenState extends State<ProfileScreen>
   @override
   void initState() {
     super.initState();
+    PostHogService.instance.screen('Profile');
 
     // Animation controller for EXP bar + stat counters
     _animController = AnimationController(