Feat: add gmail connector (#11549)

### What problem does this PR solve?

_Briefly describe what this PR aims to solve. Include background context
that will help reviewers understand the purpose of the PR._

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

web/src/locales/en.ts

@@ -739,6 +739,7 @@ Example: Virtual Hosted Style`,
         'Sync pages and databases from Notion for knowledge retrieval.',
       google_driveDescription:
         'Connect your Google Drive via OAuth and sync specific folders or drives.',
+      gmailDescription: 'Connect your Gmail via OAuth to sync emails.',
       webdavDescription: 'Connect to WebDAV servers to sync files.',
       webdavRemotePathTip:
         'Optional: Specify a folder path on the WebDAV server (e.g., /Documents). Leave empty to sync from root.',
@@ -750,6 +751,10 @@ Example: Virtual Hosted Style`,
         'Comma-separated emails whose "My Drive" contents should be indexed (include the primary admin).',
       google_driveSharedFoldersTip:
         'Comma-separated Google Drive folder links to crawl.',
+      gmailPrimaryAdminTip:
+        'Primary admin email with Gmail / Workspace access, used to enumerate domain users and as the default sync account.',
+      gmailTokenTip:
+        'Upload the OAuth JSON generated from Google Console. If it only contains client credentials, run the browser-based verification once to mint long-lived refresh tokens.',
       dropboxDescription:
         'Connect your Dropbox to sync files and folders from a chosen account.',
       dropboxAccessTokenTip: