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/components/sidecar-migration-summary.css

@@ -0,0 +1,15 @@
+/* Sidecar Migration Summary Modal — component styles only.
+   Stat cards, summary header and failure table styles are shared with the
+   metadata refresh result modal (metadata-refresh-result.css) and the batch
+   download summary modal (download-batch-summary.css); not redefined here. */
+
+.sidecar-migration-summary-modal {
+    max-width: 700px;
+}
+
+.sidecar-migration-location {
+    margin: 0 0 var(--space-3) 0;
+    font-size: var(--text-sm);
+    color: var(--text-secondary);
+    word-break: break-all;
+}