feat(ui): full frontend redesign to the Sentry-style design system

Replace the legacy UI with the dark, mono-accent design language from
design/23-sentry.html across every non-canvas surface.

- PlatformShell: fixed 252px sidebar with grouped nav (Platform /
  Operate / Workspace), brand mark, user card, and a topbar with
  search (Cmd+K palette), env badge, upgrade, notifications, and
  theme toggle; mobile collapses to a top bar with horizontal nav
- New shared component kit in shared/components/sentry.tsx:
  SPageHead, SStat, SCard, STable, SSection, SPill, Ticks, Seg,
  GhostBtn/QuietBtn/IconBtn, status tone helpers
- New DashboardPage at / — stats row, per-node CPU chart with
  1h/24h/7d ranges, resource usage tick strips, anomaly box,
  node summary table, recent deployments and project cards;
  real API data with demo-mode fixtures
- Landing page moved to /landing and rebuilt: live host telemetry
  window, install strip, feature grid, template catalog, docs
- Every page (Projects, Templates, Builds, Databases, HA,
  Operations, Activity, Security, Usage, Docs, People, Settings,
  Audit Logs, Webhooks, Admin, NodeDetail, ServiceDetail, auth,
  setup, not-found) migrated to the new kit and tokens
- Nav active state resolves by longest matching href
- Canvas internals intentionally untouched — separate pass

Verified: tsc --noEmit, eslint, vite build, Playwright walkthrough
of every route at desktop and mobile widths (demo + live session).

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

app/frontend/index.html

@@ -5,6 +5,12 @@
     <link rel="icon" type="image/svg+xml" href="/containr.svg" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
     <title>containr</title>
+    <script>
+      try {
+        document.documentElement.dataset.theme =
+          localStorage.getItem('containr.theme') === 'light' ? 'light' : 'dark';
+      } catch { document.documentElement.dataset.theme = 'dark'; }
+    </script>
   </head>
   <body>
     <div id="root"></div>