feat(app): unified demo mode + public-demo build for Vercel

- Central demo-mode helper (lib/demo-mode.ts): ?demo=1 query flag plus
  VITE_PUBLIC_DEMO build flag that makes every route demo for the
  public Vercel showcase.
- Single demo fixture source (lib/demo-data.ts): projects, services,
  builds, databases, cron, HA, security, host monitoring, templates.
  Fixes cross-page ID drift that broke project navigation.
- Demo support on Projects, HA, Security, Usage, Templates, Landing,
  Service detail; DemoRestricted gate on People/Settings/Audit/Admin.
- Demo-aware PlatformShell: nav filtering, env:demo, demo account menu,
  ?demo=1 preserved through internal links.
- TemplatesPage: selection moved into ?template= URL param for deep
  links; template fixtures shared via demo-data.
- requestJson returns {} for unparseable 200 bodies (SPA fallback) so
  envelope unwraps degrade instead of dereferencing null.
- Recharts ResponsiveContainer initialDimension kills -1 warnings.
- vercel.json: vite framework + SPA rewrites for direct deep links.

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

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

app/frontend/.gitignore

@@ -0,0 +1,2 @@
+.vercel
+.env*