system-monitor: /system-monitor route (CPU/mem/disk/processes) + page titles

- Backend GET /api/system-monitor/stats: one snapshot — CPU overall + per-core
  (two /proc/stat samples), memory + swap (/proc/meminfo), disks (df), load,
  uptime, top-20 processes (ps). Owner-only via the account gate.
- Frontend /system-monitor screen: polls every 2s; CPU/Memory/Disks cards with
  bars + per-core mini-bars, top-processes table. Nav dock "Monitor" item.
- Register /music and /system-monitor in usePageTitle RULES so the browser tab
  and editable header title update on those routes (/music was missing too).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/apps/officer-web/App.tsx

@@ -41,6 +41,7 @@ export function App() {
             <Route path="/plans" element={<Dashboard.Plans />} />
             <Route path="/files" element={<Dashboard.FilesScreen />} />
             <Route path="/music" element={<Dashboard.MusicScreen />} />
+            <Route path="/system-monitor" element={<Dashboard.SystemMonitorScreen />} />
 
             <Route path="/code-editor" element={<Dashboard.CodeEditor />} />
             <Route path="/skills" element={<Dashboard.Skills />} />