feat: make Gemini CLI project ID optional

- Remove required validation for project ID field
- Update translations to indicate field is optional (zh-CN, en)
- Auto-select first available project when left empty
- Backend already supports empty project ID by fetching project list

This improves user experience by eliminating the need to manually enter project ID for users with only one or a preferred default project.

src/pages/OAuthPage.tsx

@@ -131,12 +131,7 @@ export function OAuthPage() {
 
   const startAuth = async (provider: OAuthProvider) => {
     const projectId = provider === 'gemini-cli' ? (states[provider]?.projectId || '').trim() : undefined;
-    if (provider === 'gemini-cli' && !projectId) {
-      const message = t('auth_login.gemini_cli_project_id_required');
-      updateProviderState(provider, { projectIdError: message });
-      showNotification(message, 'warning');
-      return;
-    }
+    // 项目 ID 现在是可选的，如果不输入将自动选择第一个可用项目
     if (provider === 'gemini-cli') {
       updateProviderState(provider, { projectIdError: undefined });
     }
@@ -151,7 +146,7 @@ export function OAuthPage() {
     try {
       const res = await oauthApi.startAuth(
         provider,
-        provider === 'gemini-cli' ? { projectId: projectId! } : undefined
+        provider === 'gemini-cli' ? { projectId: projectId || undefined } : undefined
       );
       updateProviderState(provider, { url: res.url, state: res.state, status: 'waiting', polling: true });
       if (res.state) {