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/lib/query-client.ts

@@ -0,0 +1,17 @@
+import { QueryClient } from "@tanstack/react-query"
+import { ApiError } from "@/lib/api/client"
+
+export const queryClient = new QueryClient({
+  defaultOptions: {
+    queries: {
+      staleTime: 60_000,
+      gcTime: 5 * 60_000,
+      refetchOnWindowFocus: false,
+      refetchOnReconnect: false,
+      retry: (failureCount, error) => {
+        if (error instanceof ApiError && error.status < 500) return false
+        return failureCount < 2
+      },
+    },
+  },
+})