UI: activity client links, online badges, ended-leases toggle, All activity

- Activity: a client's MAC address opens its leases; "Only this" filters
  the list to that client.
- Reservations: Online is a green online / grey offline badge.
- A subnet's leases: "Include ended leases" is a checkbox.
- Dashboard: "All activity" is a button beside the heading.

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

internal/web/static/assets/style.css

@@ -353,3 +353,5 @@ table.activity td.detail { white-space: normal; color: var(--muted); }
 td.mono, span.mono { font-family: var(--mono); font-size: 13px; }
 .mt { margin-top: 10px; }
 input:disabled, select:disabled, textarea:disabled { background: var(--info-soft); color: var(--muted); cursor: not-allowed; opacity: 1; }
+.panel-head { display: flex; align-items: center; justify-content: space-between; gap: 12px; margin-bottom: 8px; }
+.panel-head h2 { margin: 0; }