feat(activities): add autocomplete & what's new popup

Features:
- Add autocomplete support for Activity search with Google Places API.
- Add "What's New" popup system to showcase new features on app update.
- Implement logic to detect fresh installs vs updates.

Fixes:
- Switch API key handling to use Firebase config for Release mode support.
- Refactor map pins to be consistent (red pins).
- UI fixes on Create Trip page (overflow issues).

Refactor:
- Make WhatsNewDialog reusable by accepting features list as parameter.

lib/services/whats_new_service.dart

@@ -0,0 +1,68 @@
+import 'package:package_info_plus/package_info_plus.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+import '../../services/logger_service.dart';
+
+class WhatsNewService {
+  static const String _lastVersionKey = 'last_known_version';
+
+  /// Vérifie si le popup "Nouveautés" doit être affiché.
+  ///
+  /// Retourne true si:
+  /// - Ce n'est PAS une nouvelle installation
+  /// - ET la version actuelle est plus récente que la version stockée
+  Future<bool> shouldShowWhatsNew() async {
+    try {
+      final prefs = await SharedPreferences.getInstance();
+      final packageInfo = await PackageInfo.fromPlatform();
+
+      final currentVersion = packageInfo.version;
+      final lastVersion = prefs.getString(_lastVersionKey);
+
+      LoggerService.info(
+        'WhatsNewService: Current=$currentVersion, Last=$lastVersion',
+      );
+
+      // Cas 1: Première installation (lastVersion est null)
+      if (lastVersion == null) {
+        // On sauvegarde la version actuelle pour ne pas afficher le popup
+        // la prochaine fois, et on retourne false maintenant.
+        await prefs.setString(_lastVersionKey, currentVersion);
+        LoggerService.info(
+          'WhatsNewService: Fresh install detected. Marking version $currentVersion as read.',
+        );
+        return false;
+      }
+
+      // Cas 2: Mise à jour (lastVersion != currentVersion)
+      if (lastVersion != currentVersion) {
+        // C'est une mise à jour, on doit afficher le popup.
+        // On NE met PAS à jour la version ici, on attend que l'utilisateur ait vu le popup.
+        LoggerService.info(
+          'WhatsNewService: Update detected ($lastVersion -> $currentVersion). Showing popup.',
+        );
+        return true;
+      }
+
+      // Cas 3: Même version
+      return false;
+    } catch (e) {
+      LoggerService.error('WhatsNewService: Error checking version: $e');
+      return false;
+    }
+  }
+
+  /// Marque la version actuelle comme "Vue".
+  /// À appeler quand l'utilisateur ferme le popup.
+  Future<void> markCurrentVersionAsSeen() async {
+    try {
+      final prefs = await SharedPreferences.getInstance();
+      final packageInfo = await PackageInfo.fromPlatform();
+      await prefs.setString(_lastVersionKey, packageInfo.version);
+      LoggerService.info(
+        'WhatsNewService: Version ${packageInfo.version} marked as seen.',
+      );
+    } catch (e) {
+      LoggerService.error('WhatsNewService: Error marking seen: $e');
+    }
+  }
+}