feat(audit): every row keeps its date; dividers for Today, Yesterday, Last 7 days, Older (#5214)

The operator found "Today 08:03" in place of a date odd on the audit log,
and asked for the recency groups as dividers instead. |when now always
writes the date ("Thu 8 Oct 08:03"); mark_day_groups gives each entry its
group in the reader's zone, and a divider row opens each group. A scrolled
page carries the group the last one ended in, so it is not opened twice.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

steward/static/steward.css

@@ -227,6 +227,9 @@ strong, b, th { font-weight: 500; }
 .table td.nowrap { white-space: nowrap; }
 /* An audit entry's details, one fact a line (milestone 490). */
 .audit-facts { font-size: 0.8rem; color: var(--text-muted); }
+/* A divider row naming the group the rows under it fall in (Today, Older). */
+.table tr.table-divider th { padding: 1rem 1rem 0.4rem; background: var(--bg-card); border-bottom: 1px solid var(--border-mid); }
+.table tbody tr.table-divider:hover th { background: var(--bg-card); }
 .cell-note { font-size: 0.78rem; color: var(--text-muted); margin-top: 0.2rem; }
 /* An alert episode's own transitions, folded under it (#5316). */
 .episode-steps { margin-top: 0.3rem; font-size: 0.8rem; }