feat: add SPA, serve at /app/, update Dockerfile and README

- React + TanStack Router + shadcn/ui SPA under spa/
- serve spa/dist at /app/ with index.html fallback for client routing
- Dockerfile: node build stage for SPA, copy dist into runtime image
- README: document SPA, CORS_ORIGINS env var, architecture entry
- vite base set to /app/, manifest.json paths fixed

spa/src/components/empty-state.tsx

@@ -0,0 +1,17 @@
+import type { LucideIcon } from "lucide-react"
+
+type EmptyStateProps = {
+  icon: LucideIcon
+  title: string
+  description?: string
+}
+
+export function EmptyState({ icon: Icon, title, description }: EmptyStateProps) {
+  return (
+    <div className="flex flex-col items-center justify-center py-16 text-center">
+      <Icon className="mb-3 size-10 text-muted-foreground/40" />
+      <p className="font-medium text-muted-foreground">{title}</p>
+      {description && <p className="mt-1 text-sm text-muted-foreground/70">{description}</p>}
+    </div>
+  )
+}