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/features/workspace/pages/ProjectWorkspacePage.tsx

@@ -19,6 +19,7 @@ import {
   type CreateServiceInput,
 } from '@/lib/api-client';
 import { getDemoProjectById, getDemoServicesByProject, getDemoVariablesByProject } from '@/lib/demo-data';
+import { useDemoMode } from '@/lib/demo-mode';
 import { useAuthSession } from '@/lib/use-auth-session';
 import { formatDate, formatRelative } from '@/lib/time';
 import type { ServiceVariable } from '../auto-connections';
@@ -466,7 +467,7 @@ export function ProjectWorkspacePage() {
   const toast = useToast();
 
   const activeView = (searchParams.get('view') as WorkspaceView | null) ?? 'canvas';
-  const isDemoMode = searchParams.get('demo') === '1';
+  const isDemoMode = useDemoMode();
   const sessionQuery = useAuthSession({ enabled: !isDemoMode });
   const signedIn = isDemoMode || Boolean(sessionQuery.data);
   const [createOpen, setCreateOpen] = useState(false);