feat(auth): add GitHub, Keycloak, Generic OAuth2 social login providers and connections page

- Fix Dropbox social login: add token_access_type=offline and userinfo compliance fix
- Add GitHub social login provider with OAuth2 registration and userinfo normalization
- Add Keycloak SSO provider with OpenID Connect discovery
- Add Generic OAuth2 provider for custom SSO integrations
- Add SAML2 SSO config fields (config only, no flow implementation)
- Add SSO auto-login: redirect directly to SSO when configured
- Add Telegram bot notification config fields
- Add admin Connections page for managing auth providers and integrations
- Add navigation links and translation keys for connections page

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

frontend/translations/en.json

@@ -617,6 +617,22 @@
   "cookie_policy.s5_p3_pre": "This Cookie Policy is part of and incorporated into our",
   "cookie_policy.s5_privacy_link": "Privacy Notice",
   "cookie_policy.s5_terms_link": "Terms of Service",
+  "connections.configure": "Configure",
+  "connections.configured": "Connected",
+  "connections.description": "Configure external authentication providers like OAuth2 and SAML.",
+  "connections.frontend_url_note": "Note: Requires Frontend URL to be configured. Configure in System Settings.",
+  "connections.linked": "Linked",
+  "connections.mobile_upload_description": "Allow users to upload files from mobile devices by scanning a QR code.",
+  "connections.mobile_upload_title": "Mobile Phone Upload",
+  "connections.qr_code_enabled": "Enable QR Code Upload",
+  "connections.save_note": "Changes to authentication providers require a restart to take effect.",
+  "connections.sso_auto_login": "Enable SSO Auto Login",
+  "connections.sso_auto_login_description": "Automatically redirect to SSO login when authentication is required.",
+  "connections.sso_auto_login_title": "SSO Auto Login",
+  "connections.title": "Connections",
+  "connections.unconfigure": "Disconnect",
+  "connections.unlinked": "Unlinked",
+  "connections.unlinked_services": "Services",
   "credentials.col_action": "Action",
   "credentials.col_credential": "Credential",
   "credentials.col_source": "Source",
@@ -1230,6 +1246,7 @@
   "nav.api_docs": "API Docs",
   "nav.api_tokens": "API Tokens",
   "nav.backup_restore": "Backup & Restore",
+  "nav.connections": "Connections",
   "nav.credentials": "Credentials",
   "nav.dark_mode": "Dark Mode",
   "nav.dashboard": "Dashboard",