feat(antigravity): 支持 Refresh Token 批量导入创建 OAuth 账号

后端新增 ValidateRefreshToken service 方法和 POST /oauth/refresh-token 端点，
前端新增 API/Composable/UI 集成，OAuthAuthorizationFlow i18n 动态化，
支持在 Antigravity 创建账号时批量粘贴 Refresh Token 自动验证并创建账号。

frontend/src/i18n/locales/en.ts

@@ -1774,13 +1774,20 @@ export default {
           authCode: 'Authorization URL or Code',
           authCodePlaceholder:
             'Option 1: Copy the complete URL\n(http://localhost:xxx/auth/callback?code=...)\nOption 2: Copy only the code parameter value',
-          authCodeHint: 'You can copy the entire URL or just the code parameter value, the system will auto-detect',
-          failedToGenerateUrl: 'Failed to generate Antigravity auth URL',
-          missingExchangeParams: 'Missing code, session ID, or state',
-          failedToExchangeCode: 'Failed to exchange Antigravity auth code'
-        }
-	      },
-      // Gemini specific (platform-wide)
+                    authCodeHint: 'You can copy the entire URL or just the code parameter value, the system will auto-detect',
+                    failedToGenerateUrl: 'Failed to generate Antigravity auth URL',
+                    missingExchangeParams: 'Missing code, session ID, or state',
+                    failedToExchangeCode: 'Failed to exchange Antigravity auth code',
+                    // Refresh Token auth
+                    refreshTokenAuth: 'Manual RT',
+                    refreshTokenDesc: 'Enter your existing Antigravity Refresh Token. Supports batch input (one per line). The system will automatically validate and create accounts.',
+                    refreshTokenPlaceholder: 'Paste your Antigravity Refresh Token...\nSupports multiple tokens, one per line',
+                    validating: 'Validating...',
+                    validateAndCreate: 'Validate & Create',
+                    pleaseEnterRefreshToken: 'Please enter Refresh Token',
+                    failedToValidateRT: 'Failed to validate Refresh Token'
+                  }
+                },      // Gemini specific (platform-wide)
       gemini: {
         helpButton: 'Help',
         helpDialog: {

frontend/src/i18n/locales/zh.ts

@@ -1913,7 +1913,15 @@ export default {
           authCodeHint: '您可以直接复制整个链接或仅复制 code 参数值，系统会自动识别',
           failedToGenerateUrl: '生成 Antigravity 授权链接失败',
           missingExchangeParams: '缺少 code / session_id / state',
-          failedToExchangeCode: 'Antigravity 授权码兑换失败'
+          failedToExchangeCode: 'Antigravity 授权码兑换失败',
+          // Refresh Token auth
+          refreshTokenAuth: '手动输入 RT',
+          refreshTokenDesc: '输入您已有的 Antigravity Refresh Token，支持批量输入（每行一个），系统将自动验证并创建账号。',
+          refreshTokenPlaceholder: '粘贴您的 Antigravity Refresh Token...\n支持多个，每行一个',
+          validating: '验证中...',
+          validateAndCreate: '验证并创建账号',
+          pleaseEnterRefreshToken: '请输入 Refresh Token',
+          failedToValidateRT: '验证 Refresh Token 失败'
         }
       },
       // Gemini specific (platform-wide)