feat(message): enhance message store management and UI stability

Implement `removeMessage` in `useMessageStore` to allow for atomic message deletion from the state. Improve `useChatScreen` loading logic to prevent layout jumps and ensure consistent UI state during initial message loading. Additionally, add type safety for WebSocket sequence numbers and optimize message selector stability.

- Add `removeMessage` action to `useMessageStore` for state-consistent deletions
- Refine `useChatScreen` loading state logic to prevent FlashList layout issues
- Ensure `useMessages` hook returns a stable `EMPTY_MESSAGES` constant
- Cast WebSocket `seq` to integer to ensure consistent numeric comparison
- Update `useChatScreen` to use `useMessageStore` for immediate local deletion feedback

src/stores/message/store.ts

@@ -85,6 +85,7 @@ export interface MessageActions {
   removeConversation: (conversationId: string) => void;
   setMessages: (conversationId: string, messages: MessageResponse[]) => void;
   mergeMessages: (conversationId: string, incoming: MessageResponse[]) => void;
+  removeMessage: (conversationId: string, messageId: string) => void;
   patchMessages: (conversationId: string, patches: Map<string, Partial<MessageResponse>>) => void;
   setUnreadCount: (total: number, system: number) => void;
   setLastSystemMessageAt: (time: string | null) => void;
@@ -349,6 +350,19 @@ export const useMessageStore = create<MessageStore>((set, get) => ({
     });
   },
 
+  removeMessage: (conversationId: string, messageId: string) => {
+    const normalizedId = normalizeConversationId(conversationId);
+    set(state => {
+      const existing = state.messagesMap.get(normalizedId);
+      if (!existing) return state;
+      const filtered = existing.filter(m => String(m.id) !== String(messageId));
+      if (filtered.length === existing.length) return state;
+      const newMessagesMap = new Map(state.messagesMap);
+      newMessagesMap.set(normalizedId, filtered);
+      return { messagesMap: newMessagesMap };
+    });
+  },
+
   /**
    * 原子性 patch：按消息 ID 更新指定字段（如 sender、status）
    * 不影响其他消息，不丢失并发写入