feat: set up Flutter project with drift database, Riverpod providers, and LLM service layer

- Configure dependencies: drift 2.32.x, flutter_riverpod 3.3.1, fl_chart 1.2.x,
  flutter_local_notifications 21.x, local_auth 3.x, and supporting packages
- Drop riverpod_generator (incompatible analyzer version with drift_dev 2.32.x);
  convert all providers to manual Riverpod 3.x API using Notifier<T> and typed families
- Define drift schema: MoodEntries, Activities, EntryActivities, Tags, EntryTags,
  LlmInsights, UserSettings with four DAOs (mood, activity, insight, settings)
- Add LLM service abstraction (LlmService) and MediaPipe implementation
- Fix SettingsDao.delete → deleteByKey to avoid shadowing drift's inherited delete method

lib/core/llm/llm_service.dart

@@ -0,0 +1,5 @@
+abstract class LlmService {
+  Future<void> initialise();
+  Stream<String> generateStream(String prompt);
+  void dispose();
+}

lib/core/llm/mediapipe_llm_service.dart

@@ -0,0 +1,30 @@
+import 'package:flutter_mediapipe_chat/flutter_mediapipe_chat.dart';
+import 'package:path_provider/path_provider.dart';
+import 'package:path/path.dart' as p;
+import 'llm_service.dart';
+
+class MediapipeLlmService implements LlmService {
+  final _chat = FlutterMediapipeChat();
+  bool _initialised = false;
+
+  static const _modelFileName = 'gemma-2b-it-cpu-int4.bin';
+
+  @override
+  Future<void> initialise() async {
+    if (_initialised) return;
+    final dir = await getApplicationSupportDirectory();
+    final modelPath = p.join(dir.path, _modelFileName);
+    await _chat.loadModel(ModelConfig(path: modelPath));
+    _initialised = true;
+  }
+
+  @override
+  Stream<String> generateStream(String prompt) {
+    return _chat.generateResponseAsync(prompt);
+  }
+
+  @override
+  void dispose() {
+    _initialised = false;
+  }
+}