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/hooks/useWatchlist.js

@@ -64,7 +64,7 @@ export const useWatchlist = () => {
     const handleAddToWatchlist = useCallback(async (stockCode, stockName) => {
         try {
             const base = getApiBase();
-            const resp = await fetch(base + '/api/account/watchlist/add', {
+            const resp = await fetch(base + '/api/account/watchlist', {
                 method: 'POST',
                 credentials: 'include',
                 headers: { 'Content-Type': 'application/json' },