feat: service categories end-to-end + monitoring widgets on main dashboard

Service categories (described in README roadmap, never wired):
- Backend: POST /services now persists category/containerId/port/ip/tailscaleOnly
- Backend: POST /services/update accepts category for in-place changes
- Frontend: category <select> in add-service modal (local + external)
- Frontend: category <select> in edit-service modal with current value
- Frontend: All Categories dropdown in service filter bar (auto-populated
  from both API categories and any categories present on rendered cards)
- Frontend: colored category badge (icon + name) on service cards
- Frontend: filter auto-refreshes after buildGrid

Monitoring on main dashboard (replaces orphaned monitoring-dashboard.html):
- New monitoring-widgets.js embeds a 5-card System Overview panel above
  the filter bar: Services, Containers Up, Avg CPU, Avg Memory, Health
- Pulls /api/v1/monitoring/stats + /api/v1/health-checks/status
- Auto-refreshes on DC.POLL.STATS (5s), color-coded bars (warn >=65%, bad >=85%)

Build:
- Added monitoring-widgets.js to init.js bundle in build.js
- Rebuilt dist/ bundles (core.js, features.js, init.js)
- sw.js cache version bumped automatically
- CSP hash regenerated

status/js/core/service-create.js

@@ -262,6 +262,7 @@
     const proxyIp = document.getElementById('external-proxy-ip').value.trim() || SITE.dnsIp || 'localhost';
     const preserveHost = document.getElementById('external-preserve-host').checked;
     const followRedirects = document.getElementById('external-follow-redirects').checked;
+    const category = document.getElementById('external-service-category')?.value || '';
 
     if (!name || !externalUrl) {
       showNotification('Please fill in Name and External URL', 'warning');
@@ -341,6 +342,8 @@
         isExternal: true,
         isCustom: true
       };
+      // Only attach category if user actually picked one
+      if (category) newService.category = category;
 
       window.APPS.push(newService);
       results.dashboard = true;
@@ -457,6 +460,13 @@
     const healthCheck = document.getElementById('health-check-input')?.value || '';
     const timeout = document.getElementById('timeout-input')?.value || 30;
 
+    // Category is optional — pulled from either local or external select by the
+    // openAddServiceModal reset. If user doesn't choose one, it stays undefined
+    // and we don't send it (so the backend keeps the existing behavior).
+    const categoryEl = document.getElementById('service-category-input')
+      || document.getElementById('external-service-category');
+    const category = categoryEl?.value || '';
+
     const dnsToken = window.getToken(getPrimaryDnsId(), 'admin');
 
     if (!name || !port || !ip) {
@@ -525,6 +535,8 @@
         logo: logo || `/assets/${subdomain}.png`,
         tailscaleOnly: tailscaleOnly || false
       };
+      // Only include category if user actually picked one
+      if (category) serviceConfig.category = category;
 
       await window.addServiceToConfig(serviceConfig);
       results.dashboard = true;