Closes the last buildable item of the #411 system-playlists-v2
umbrella. System playlists atomic-replace on rebuild, so created_at
(already on the wire — no server change) is the last-rotated time.
Surface it as a small tile subtitle so users see how fresh a mix
is: "Refreshed just now / today / yesterday / N days ago / Mon D".
- web PlaylistCard: refreshedLabel() + a muted footer line, shown
only when system_variant != null. Unparseable/empty timestamp →
suppressed (web test fixtures use created_at:'' so no test churn).
- flutter PlaylistCard: mirrored _refreshedLabel() + subtitle under
the system badge for isSystem playlists.
Friendly wording deliberately distinct from HistoryRow's "m/h ago";
per-surface helper per the project's existing relative-time
convention. CI-pending; closes with the umbrella on device-check.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
bvandeusen
ce36760819