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/public/containr.svg

@@ -1,12 +1,12 @@
 <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 64 64">
   <defs>
     <linearGradient id="bg" x1="0" y1="0" x2="64" y2="64" gradientUnits="userSpaceOnUse">
-      <stop offset="0" stop-color="#cdf06f"/>
-      <stop offset="1" stop-color="#7fb524"/>
+      <stop offset="0" stop-color="#56c9f8"/>
+      <stop offset="1" stop-color="#0e86c9"/>
     </linearGradient>
   </defs>
   <rect width="64" height="64" rx="15" fill="url(#bg)"/>
-  <g fill="none" stroke="#131507" stroke-width="4.2" stroke-linecap="round" stroke-linejoin="round">
+  <g fill="none" stroke="#061018" stroke-width="4.2" stroke-linecap="round" stroke-linejoin="round">
     <rect x="15" y="18" width="34" height="26" rx="4"/>
     <path d="M23 18v26M31 18v26M39 18v26"/>
     <path d="M20 51h24"/>