feat(sidecars): restyle migration result as a summary modal

Follow the app's existing operation-summary convention (Metadata Fetch
Summary / Batch Download Summary): a self-managed modal appended to
document.body with a 3-state summary header, stat cards (moved /
models / skipped / conflicts / errors), and a failure table listing
per-model errors that were previously swallowed into a single count.

The storage location line and Open Folder action move into the modal
actions; the page reload still happens only when the modal is
dismissed. ESC is captured so it never reaches the settings modal
underneath. The obsolete migrateSuccess toast key is dropped — the
modal is the success feedback now.

static/css/style.css

@@ -43,6 +43,7 @@
 @import 'components/media-viewer.css';
 @import 'components/metadata-refresh-result.css';
 @import 'components/download-batch-summary.css';
+@import 'components/sidecar-migration-summary.css';
 
 .initialization-notice {
     display: flex;