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-modals.js

@@ -82,6 +82,16 @@
             Enter a URL or upload an image file (PNG, JPG, SVG)
           </div>
         </div>
+
+        <!-- Category -->
+        <div>
+          <label for="edit-service-category" class="form-label-accent-sm">
+            Category
+          </label>
+          <select id="edit-service-category" data-role="service-category" class="form-input-md">
+            <option value="">— No category —</option>
+          </select>
+        </div>
       </div>
 
       <div class="weather-modal-buttons" style="margin-top: 24px;">
@@ -239,6 +249,15 @@
               Reload Caddy after adding
             </label>
 
+            <!-- Category -->
+            <div>
+              <label for="service-category-input" style="font-size: 0.8rem; color: var(--muted); margin-bottom: 4px; display: block;">Category</label>
+              <select id="service-category-input" data-role="service-category" style="width: 100%;">
+                <option value="">— No category —</option>
+              </select>
+              <div style="font-size: 0.7rem; color: var(--muted); margin-top: 3px;">Group services on the dashboard by purpose (Media, Productivity, etc.)</div>
+            </div>
+
             <hr style="border: none; border-top: 1px solid var(--border); margin: 4px 0;" />
 
             <div class="grid-2col">
@@ -326,6 +345,14 @@
                 Follow Redirects
               </label>
 
+              <!-- Category (external) -->
+              <div>
+                <label for="external-service-category" style="font-size: 0.8rem; color: var(--muted); margin-bottom: 4px; display: block;">Category</label>
+                <select id="external-service-category" data-role="service-category" style="width: 100%;">
+                  <option value="">— No category —</option>
+                </select>
+              </div>
+
             </div>
           </details>
         </div>