Add WeChat login binding gate and context-aware auth UX.

Stop auto-creating duplicate accounts on OAuth, add bind-or-register gate,
PC scan login, mobile open-in-WeChat guide, and fix localhost session cookies.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/components/ChatView.tsx

@@ -7,7 +7,8 @@ import { openAvatarPicker } from '../utils/userAvatar';
 import { HistorySidebar } from './HistorySidebar';
 import { TKMindAvatar } from './TKMindAvatar';
 import { ChatPanel } from './ChatPanel';
-import { BalanceRing } from './BalanceRing';
+import { WechatBindPrompt } from './WechatBindPrompt';
+import { WechatAccountButton } from './WechatAccountButton';
 import type { MindSpaceSaveCategory } from '../types';
 
 export function ChatView({
@@ -144,6 +145,7 @@ export function ChatView({
             <button type="button" className="ghost-btn" onClick={() => void handleNewSession()}>
               新会话
             </button>
+            {user && <WechatAccountButton returnTo={window.location.pathname} />}
             {onLogout && (
               <button type="button" className="ghost-btn logout-btn" onClick={onLogout}>
                 登出
@@ -156,6 +158,8 @@ export function ChatView({
           <div className="banner banner-offline">网络已断开，请检查连接后重试</div>
         )}
 
+        {user && <WechatBindPrompt returnTo={window.location.pathname} />}
+
         {typeof balanceCents === 'number' && balanceCents <= 0 && (
           <div className="banner banner-error">
             <span>余额不足，请充值后继续使用</span>