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.
