1.6.1

Small operational patch: handle Google OAuth invalid_grant refresh failures gracefully and force-refresh client cache for Google credentials.

Backend:
Gracefully handle Google invalid_grant errors during token refresh by returning a TokenRefreshResponse that includes needs_reauth: true and an auth_url placeholder, instead of returning HTTP 400. This lets the frontend prompt for re-authorization without noisy errors.
Added defensive handling around token refresh in the Google endpoints across service_invoice, service_plenty, service_omni, and service_jtl.
Extended the TokenRefreshResponse model with auth_url and needs_reauth fields to avoid schema mismatches with the frontend.
Frontend:
The credentials "Refresh" action now forces a server fetch (bypassing local cache) to surface the latest credential state after re-authorization or auto-refresh attempts.
Auto-refresh flow treats a refresh response containing auth_url as a signal that manual re-authorization is required, and surfaces the auth dialog.

Fix a bug where token refresh failures caused the backend to raise HTTP 400 errors for invalid_grant, which prevented the frontend from handling re-authorization flows automatically.
Fix UI inconsistency where the refresh button could return stale results due to cached credentials.