feat(chat): LLM 动态对话建议 — 替换硬编码关键词匹配

AI 回复结束后，将最近对话发给 LLM 生成 3 个上下文相关的后续问题，
替换原有的"继续深入分析"等泛泛默认建议。

变更:
- llm-service.ts: 添加 suggestions 提示模板 + llmSuggest() 辅助函数
- streamStore.ts: SSE 流式请求 via SaaS relay，response.text() 一次性
  读取避免 Tauri WebView2 ReadableStream 兼容问题，失败降级到关键词
- chatStore.ts: suggestionsLoading 状态镜像
- SuggestionChips.tsx: loading 骨架动画
- ChatArea.tsx: 传递 loading prop

desktop/src/store/chatStore.ts

@@ -79,6 +79,7 @@ interface ChatState {
   totalOutputTokens: number;
   chatMode: ChatModeType;
   suggestions: string[];
+  suggestionsLoading: boolean;
 
   addMessage: (message: Message) => void;
   updateMessage: (id: string, updates: Partial<Message>) => void;
@@ -111,6 +112,7 @@ export const useChatStore = create<ChatState>()(
   isLoading: false,
   chatMode: 'thinking' as ChatModeType,
   suggestions: [],
+  suggestionsLoading: false,
   totalInputTokens: 0,
   totalOutputTokens: 0,
 
@@ -367,6 +369,7 @@ const unsubStream = useStreamStore.subscribe((state) => {
   if (chat.isLoading !== state.isLoading) updates.isLoading = state.isLoading;
   if (chat.chatMode !== state.chatMode) updates.chatMode = state.chatMode;
   if (chat.suggestions !== state.suggestions) updates.suggestions = state.suggestions;
+  if (chat.suggestionsLoading !== state.suggestionsLoading) updates.suggestionsLoading = state.suggestionsLoading;
   if (Object.keys(updates).length > 0) {
     useChatStore.setState(updates);
   }