feat(stats): roll up same-episode sessions within a day

Group sessions for the same video inside each day header so repeated
viewings of an episode collapse into a single expandable bucket with
aggregate active time and card counts. Multi-session buckets get a
dedicated delete action that wipes every session in the group via the
bulk delete endpoint; singleton buckets continue to render the existing
SessionRow flow unchanged.

Delete confirmation copy lives in delete-confirm for parity with the
other bulk-delete flows, and the bucket delete path is extracted into a
pure buildBucketDeleteHandler factory so it can be unit-tested without
rendering the tab. MediaSessionList is intentionally untouched -- it is
already scoped to a single video and doesn't need rollup.

stats/src/lib/delete-confirm.ts

@@ -17,3 +17,9 @@ export function confirmAnimeGroupDelete(title: string, count: number): boolean {
 export function confirmEpisodeDelete(title: string): boolean {
   return globalThis.confirm(`Delete "${title}" and all its sessions?`);
 }
+
+export function confirmBucketDelete(title: string, count: number): boolean {
+  return globalThis.confirm(
+    `Delete all ${count} session${count === 1 ? '' : 's'} of "${title}" from this day?`,
+  );
+}