Add 10 UX/feature improvements across dashboard, status page, and API

- Incident acknowledge: Ack button in dashboard rail and ResourceDetail incidents table
- Status page 90-day uptime history bars (teal ≥99%, amber ≥95%, red <95%, grey = no data)
- Theme setting applied: data-theme attribute wired to saved setting with CSS variables + light mode overrides
- Sparkline replaced with colored status bars (teal=up, red=down, grey=unknown) per check tick
- ResourceDetail chart time-range selector: 1h / 6h / 24h / 7d toggle fetches from checks endpoint
- Bulk actions: checkbox per row, select-all header, bulk delete and maintenance toggle bar
- CSV export adds 6 new fields (http_method, http_body, heartbeat_timeout, is_public, consecutive_failures_threshold, response_time_threshold)
- CSV import handles those 6 new fields on INSERT
- Check detail expansion: click expandable rows in Recent Checks to show full details JSON
- Browser push notifications: request permission opt-in, new Notification() on WS message when tab unfocused

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/src/NotificationCenter.css

@@ -277,6 +277,11 @@
   background: #475569;
 }
 
+/* ── Push prompt ── */
+.push-prompt {
+  padding: 0.4rem 0.875rem 0;
+}
+
 /* ── Mobile ── */
 @media (max-width: 600px) {
   .notification-popup {