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/build/asset-manifest.json

@@ -1,15 +1,15 @@
 {
   "files": {
-    "main.css": "/static/css/main.5afb44a9.css",
-    "main.js": "/static/js/main.a387bcf0.js",
+    "main.css": "/static/css/main.67d2dbc4.css",
+    "main.js": "/static/js/main.85e69234.js",
     "static/js/453.d456ce70.chunk.js": "/static/js/453.d456ce70.chunk.js",
     "index.html": "/index.html",
-    "main.5afb44a9.css.map": "/static/css/main.5afb44a9.css.map",
-    "main.a387bcf0.js.map": "/static/js/main.a387bcf0.js.map",
+    "main.67d2dbc4.css.map": "/static/css/main.67d2dbc4.css.map",
+    "main.85e69234.js.map": "/static/js/main.85e69234.js.map",
     "453.d456ce70.chunk.js.map": "/static/js/453.d456ce70.chunk.js.map"
   },
   "entrypoints": [
-    "static/css/main.5afb44a9.css",
-    "static/js/main.a387bcf0.js"
+    "static/css/main.67d2dbc4.css",
+    "static/js/main.85e69234.js"
   ]
 }