refactor(auth): remove redirect_uri parameter from OAuth flow

Simplify OAuth login and account linking by removing the redirect_uri
parameter from initiateLogin and initiateLink functions. The backend
now handles callback URL construction internally.

src/pages/auth/LoginPage.tsx

@@ -512,14 +512,9 @@ export default function LoginPage() {
     setIsLoading(true);
 
     try {
-      // The redirect_uri Google will call is the *backend* callback.
-      // The backend then redirects to the frontend /oauth/callback with the token.
-      const backendCallbackUri = `${import.meta.env.VITE_API_BASE_URL ?? 'http://localhost:5000/api/v1'}/auth/external/${provider}/callback`;
-
       // Ask backend for the Google authorization URL
       // If we're in an OIDC bridge flow, pass oidc_session_id so it survives the round-trip
       const response = await api.externalAuth.initiateLogin(provider, {
-        redirect_uri: backendCallbackUri,
         flow: 'login',
         ...(oidcSessionId ? { oidc_session_id: oidcSessionId } : {}),
       });