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/index.html

@@ -256,6 +256,9 @@
       <option value="on">🟢 Online</option>
       <option value="off">🔴 Offline</option>
     </select>
+    <select id="service-filter-category" style="padding: 8px 12px; background: var(--bg); border: 1px solid var(--border); border-radius: 6px; color: var(--fg); font-size: 0.9rem;">
+      <option value="all">All Categories</option>
+    </select>
     <button id="batch-operations-btn" class="btn-sm" style="padding: 8px 12px;">☰ Batch Operations</button>
     <span id="service-filter-count" style="color: var(--muted); font-size: 0.85rem; white-space: nowrap;"></span>
   </div>