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/message-content.tsx

@@ -29,7 +29,7 @@ export function MessageContent({ content, imageUrl, isFromAI }: MessageContentPr
       style={{
         display: "flex",
         flexDirection: "column",
-        // **关键**：在 row flex 父中撑开（不依赖 content 循环）—— 让 TextBubble 能拿到真实宽度
+        // 关键：在 row flex 父中撑开（不依赖 content 循环）—— 让 TextBubble 能拿到真实宽度
         flex: "1 1 0",
         // 防超长 content（图片 / 超长 url）撑爆父
         minWidth: 0,