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

dashcaddy-api/routes/services.js

@@ -372,7 +372,7 @@ module.exports = function({
   // Add a new service
   router.post('/services', asyncHandler(async (req, res) => {
     try {
-      const { id, name, logo } = req.body;
+      const { id, name, logo, category, containerId, port, ip, tailscaleOnly } = req.body;
 
       if (!id || !name) {
         throw new ValidationError('id and name are required');
@@ -391,7 +391,14 @@ module.exports = function({
           throw new ConflictError(`Service "${id}" already exists`, id);
         }
 
-        services.push({ id, name, logo: logo || `/assets/${id}.png` });
+        const newService = { id, name, logo: logo || `/assets/${id}.png` };
+        // Persist optional metadata fields if provided
+        if (category) newService.category = category;
+        if (containerId) newService.containerId = containerId;
+        if (port) newService.port = port;
+        if (ip) newService.ip = ip;
+        if (typeof tailscaleOnly === 'boolean') newService.tailscaleOnly = tailscaleOnly;
+        services.push(newService);
         return services;
       });
 
@@ -542,6 +549,8 @@ module.exports = function({
           };
           if (name) services[serviceIndex].name = name;
           if (logo) services[serviceIndex].logo = logo;
+          // Allow category update via update endpoint too (optional body field)
+          if (req.body.category !== undefined) services[serviceIndex].category = req.body.category || undefined;
           results.services = 'updated';
         } else {
           results.services = 'not found';