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

docsgpt/core/settings.py

@@ -251,6 +251,9 @@ class Settings(BaseSettings):
     CONNECTOR_REDIRECT_BASE_URI: Optional[str] = (
         "http://127.0.0.1:7091/api/connectors/callback"  ##add redirect url as it is to your provider's console(gcp)
     )
+    # Comma-separated frontend origins allowed to receive connector OAuth results, e.g. https://docsgpt.example.com.
+    # The callback origin and OIDC_FRONTEND_URL are always allowed; a loopback callback also allows localhost:5173.
+    CONNECTOR_ALLOWED_ORIGINS: Optional[str] = None
 
     # Microsoft Entra ID (Azure AD) integration
     MICROSOFT_CLIENT_ID: Optional[str] = None  # Azure AD Application (client) ID