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/sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'dashcaddy-shell-8ef9c82616';
+const CACHE = 'dashcaddy-shell-43a872cc40';
 const PRECACHE = [
   '/',
   '/index.html',