When user attempts to change model storage location with no models
downloaded, the migration API returns moved=0 early. Previously the UI
would still call setCustomModelsDir() and restart the server, causing
unexpected behavior (hang/connection lost).
This change checks migrationResult.moved === 0 and shows a helpful
toast message instead of proceeding with the storage change.
Fixes: #426
Co-authored-by: fuleinist <[email protected]>
