fix: stop leaking connector OAuth session tokens to other origins

The connector OAuth popup posted the session token to window.opener with
a '*' target origin, so any page that opened the popup received it. An
attacker with an account on a multi-user deployment could start a flow for
their own pending session, get a victim to finish the provider consent, and
receive a token backed by the victim's Drive/SharePoint/Confluence tokens.

- Post popup results only to allowed frontend origins: the callback origin,
  OIDC_FRONTEND_URL, the new CONNECTOR_ALLOWED_ORIGINS, and localhost:5173
  when the callback runs on a loopback host.
- Render the success page from the callback itself so the token never
  appears in a URL; callback-status ignores session_token/user_email params.
- ConnectorAuth accepts messages only from the popup it opened, on the
  callback origin reported by /api/connectors/auth.
- /api/connectors/disconnect requires auth and only deletes the caller's
  session.
- /api/connectors/sync and /api/remote reject session tokens the caller
  does not own.

Fixes #2766

docs/content/Guides/Integrations/confluence-connector.mdx

@@ -52,6 +52,7 @@ VITE_CONFLUENCE_CLIENT_ID=your-atlassian-client-id
 | `CONFLUENCE_CLIENT_ID` | Client ID from your Atlassian OAuth app | Yes |
 | `CONFLUENCE_CLIENT_SECRET` | Client secret from your Atlassian OAuth app | Yes |
 | `VITE_CONFLUENCE_CLIENT_ID` | Same Client ID, used by the frontend to show the Confluence option | Yes |
+| `CONNECTOR_ALLOWED_ORIGINS` | Comma-separated frontend origins allowed to receive the sign-in result, e.g. `https://docsgpt.example.com`. Not needed when the frontend shares the API origin or runs on `localhost:5173` | When the frontend is on its own origin |
 
 ### Step 5: Restart and Use
 
@@ -62,6 +63,7 @@ Restart your application, then go to the upload section in DocsGPT and select **
 ## Troubleshooting
 
 - **Option not appearing** — Verify `VITE_CONFLUENCE_CLIENT_ID` is set in the frontend `.env`, then restart.
+- **Sign-in popup closes but the account never connects** — The frontend origin is not allowed to receive the result. Add it to `CONNECTOR_ALLOWED_ORIGINS` in the backend `.env`.
 - **Authentication failed** — Check that the callback URL matches exactly, including `?provider=confluence`.
 - **No accessible sites** — Ensure the authenticating user has access to at least one Confluence Cloud site.
 - **Permission denied** — Verify that the Confluence API scopes are enabled in your Atlassian app settings.

docs/content/Guides/Integrations/google-drive-connector.mdx

@@ -50,6 +50,7 @@ VITE_GOOGLE_CLIENT_ID=your-google-client-id
 | `GOOGLE_CLIENT_ID` | OAuth Client ID from GCP Credentials | Yes |
 | `GOOGLE_CLIENT_SECRET` | OAuth Client Secret from GCP Credentials | Yes |
 | `VITE_GOOGLE_CLIENT_ID` | Same Client ID, used by the frontend to show the Google Drive option | Yes |
+| `CONNECTOR_ALLOWED_ORIGINS` | Comma-separated frontend origins allowed to receive the sign-in result, e.g. `https://docsgpt.example.com`. Not needed when the frontend shares the API origin or runs on `localhost:5173` | When the frontend is on its own origin |
 
 <Callout type="warning" emoji="⚠️">
 Make sure to use the same Google Client ID in both backend and frontend configurations.
@@ -65,6 +66,7 @@ Restart your application, then go to the upload section in DocsGPT and select **
 
 - **Option not appearing** — Verify `VITE_GOOGLE_CLIENT_ID` is set in the frontend `.env`, then restart.
 - **Authentication failed** — Check that the redirect URI matches exactly, including `?provider=google_drive`. Ensure the Google Drive API is enabled.
+- **Sign-in popup closes but the account never connects** — The frontend origin is not allowed to receive the result. Add it to `CONNECTOR_ALLOWED_ORIGINS` in the backend `.env`.
 - **Permission denied** — Verify the OAuth consent screen is configured and the user has access to the target files.
 - **Files not processing** — Check backend logs and verify that backend environment variables are correctly set.
 

docs/content/Guides/Integrations/sharepoint-connector.mdx

@@ -45,6 +45,7 @@ MICROSOFT_TENANT_ID=your-azure-ad-tenant-id
 | `MICROSOFT_CLIENT_SECRET` | Client secret value | Yes | — |
 | `MICROSOFT_TENANT_ID` | Directory (tenant) ID | No | `common` |
 | `MICROSOFT_AUTHORITY` | Login endpoint override | No | Auto-constructed |
+| `CONNECTOR_ALLOWED_ORIGINS` | Comma-separated frontend origins allowed to receive the sign-in result, e.g. `https://docsgpt.example.com` | When the frontend is on its own origin | API origin and `localhost:5173` |
 
 <Callout type="warning">
 `MICROSOFT_TENANT_ID=common` (the default) allows any Microsoft account to authenticate. Set this to your specific tenant ID in production.
@@ -60,4 +61,5 @@ Restart your application, then go to the upload section in DocsGPT and select **
 
 - **Option not appearing** — Verify `MICROSOFT_CLIENT_ID` and `MICROSOFT_CLIENT_SECRET` are set, then restart.
 - **Authentication failed** — Check that the redirect URI matches exactly, including `?provider=share_point`.
+- **Sign-in popup closes but the account never connects** — The frontend origin is not allowed to receive the result. Add it to `CONNECTOR_ALLOWED_ORIGINS` in the backend `.env`.
 - **Permission denied** — Ensure admin consent is granted and the user has access to the target files.