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/src/lib/api-client.ts

@@ -424,7 +424,10 @@ async function requestJson<T>(path: string, init?: RequestInit): Promise<T> {
     throw new ApiError(message, response.status);
   }
 
-  return payload as T;
+  // SPA fallback servers may answer /api/* with index.html — an ok response
+  // whose body is not JSON. Surface an empty object so envelope unwraps
+  // (`payload.x ?? []`) degrade instead of dereferencing null.
+  return (payload ?? {}) as T;
 }
 
 async function requestText(path: string, init?: RequestInit): Promise<string> {