style: remove markdown bold syntax from code comments

Remove `**...**` emphasis markers from inline comments and JSDoc blocks across
auth and chat components, machine mappers, and quota helpers. These are
plain code comments, not rendered markdown, so the bold syntax was noise.

Files touched:
- src/app/auth/components/auth-screen.tsx
- src/app/chat/components/chat-header.tsx
- src/app/chat/components/chat-screen.tsx
- chat machine mapper / quota helpers

No functional or behavioral changes.

src/app/chat/components/chat-header.tsx

@@ -8,7 +8,7 @@
  *   - 游客模式（isGuest=true）：渲染 `GuestBanner` 提示注册（点击 → /auth）
  *   - 登录模式（isGuest=false）：渲染 `MenuButton`（点击 → push 到 /sidebar）
  *
- * 注意：菜单按钮直接 push 到侧边栏路由，**不**打开下拉菜单
+ * 注意：菜单按钮直接 push 到侧边栏路由，不打开下拉菜单
  * （Dart 端 `MenuButton(onTap: _goSidebar)` 行为）。
  */
 import { useRouter } from "next/navigation";