fix: 修复自选股添加失败 405 错误

- useWatchlist.js: 修正 API 路径从 /api/account/watchlist/add 改为 /api/account/watchlist
- account.js: 同步修改 mock handler 路径

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/mocks/handlers/account.js

@@ -159,7 +159,7 @@ export const accountHandlers = [
   }),
 
   // 6. 添加自选股
-  http.post('/api/account/watchlist/add', async ({ request }) => {
+  http.post('/api/account/watchlist', async ({ request }) => {
     await delay(NETWORK_DELAY);
 
     const currentUser = getCurrentUser();