refactor(database): migrate to new modular database layer and unify data access

- Remove legacy database.ts, LocalDataSource.ts, and MessageRepository.ts
- Create new src/database/ module with messageRepository, userCacheRepository, conversationRepository, and groupCacheRepository
- Update all consumers to import from @/database instead of services/database
- Add web platform blur handling for modal components to fix focus issues
- Flatten SystemMessageItem and NotificationsScreen styles for consistent design
- Add draggable slider in ChatSettingsScreen and dynamic font size support
- Introduce 9 new chat color themes
- Add profile screens for about, terms, and privacy policy with navigation routes
- Add policy links to login and registration screens
- Fix post share URL format from /posts/ to /post/

src/database/schema/ConversationSchema.ts

@@ -0,0 +1,40 @@
+import type { SQLiteDatabase } from 'expo-sqlite';
+
+export const CONVERSATIONS_TABLE = 'conversations';
+export const CONVERSATION_CACHE_TABLE = 'conversation_cache';
+export const CONVERSATION_LIST_CACHE_TABLE = 'conversation_list_cache';
+
+export const CREATE_CONVERSATIONS_TABLE = `CREATE TABLE IF NOT EXISTS conversations (
+  id TEXT PRIMARY KEY NOT NULL,
+  participantId TEXT NOT NULL,
+  lastMessageId TEXT,
+  lastSeq INTEGER DEFAULT 0,
+  unreadCount INTEGER DEFAULT 0,
+  createdAt TEXT NOT NULL,
+  updatedAt TEXT NOT NULL
+)`;
+
+export const CREATE_CONVERSATION_CACHE_TABLE = `CREATE TABLE IF NOT EXISTS conversation_cache (
+  id TEXT PRIMARY KEY NOT NULL,
+  data TEXT NOT NULL,
+  updatedAt TEXT NOT NULL
+)`;
+
+export const CREATE_CONVERSATION_LIST_CACHE_TABLE = `CREATE TABLE IF NOT EXISTS conversation_list_cache (
+  id TEXT PRIMARY KEY NOT NULL,
+  data TEXT NOT NULL,
+  updatedAt TEXT NOT NULL
+)`;
+
+export const MIGRATE_CONVERSATIONS_TABLE = async (db: SQLiteDatabase): Promise<void> => {
+  try {
+    const tableInfo = await db.getAllAsync<{ name: string }>(`PRAGMA table_info(conversations)`);
+    const columns = tableInfo.map(col => col.name);
+
+    if (!columns.includes('lastSeq')) {
+      await db.execAsync(`ALTER TABLE conversations ADD COLUMN lastSeq INTEGER DEFAULT 0`);
+    }
+  } catch (error) {
+    console.error('[ConversationSchema] 迁移失败:', error);
+  }
+};