feat(message): implement incremental sync and atomic unread updates

Refactor the messaging system to improve synchronization efficiency and state consistency.

- Implement incremental message synchronization using sequence numbers (seq) to reduce payload size.
- Add `markAllAsRead` batch API support to optimize read receipt processing.
- Introduce atomic state updates in `useMessageStore` to prevent inconsistent UI rendering during unread count changes.
- Implement notification deduplication in `WSMessageHandler` to prevent duplicate unread increments during reconnection.
- Optimize `MessageSyncService` to prioritize incremental sync over full snapshots when local data exists.
- Refactor `ReadReceiptManager` to use optimistic updates with proper rollback mechanisms.
- Fix minor UI issues in `SettingsScreen` and `PrivacySettingsScreen` related to theme picker z-index and style application.

src/services/message/messageService.ts

@@ -568,6 +568,19 @@ class MessageService {
     });
   }
 
+  /**
+   * 批量标记所有会话已读
+   * POST /api/v1/conversations/read-all
+   */
+  async markAllAsRead(conversations: Array<{ id: string; lastSeq: number }>): Promise<void> {
+    await api.post('/conversations/read-all', {
+      conversations: conversations.map(c => ({
+        conversation_id: c.id,
+        last_read_seq: Number(c.lastSeq),
+      })),
+    });
+  }
+
   /**
    * 上报输入状态
    * 优先使用WebSocket，失败时降级到HTTP
@@ -597,6 +610,15 @@ class MessageService {
     }
   }
 
+  /**
+   * 获取同步元数据（轻量级 seq + 时间，用于增量同步判断）
+   * GET /api/v1/conversations/sync-data
+   */
+  async getSyncData(): Promise<Array<{ id: string; max_seq: number; last_message_at: string }>> {
+    const response = await api.get<{ conversations: Array<{ id: string; max_seq: number; last_message_at: string }> }>('/conversations/sync-data');
+    return response.data?.conversations || [];
+  }
+
   /**
    * 获取单个会话未读数
    * GET /api/v1/conversations/unread/count?conversation_id=xxx