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/rating-histogram.tsx

@@ -0,0 +1,26 @@
+type RatingHistogramProps = {
+  histogram: number[]
+}
+
+export function RatingHistogram({ histogram }: RatingHistogramProps) {
+  const max = Math.max(...histogram, 1)
+
+  return (
+    <div>
+      <div className="flex items-end gap-1" style={{ height: 40 }}>
+        {histogram.map((count, i) => (
+          <div
+            key={i}
+            className="flex-1 rounded-t bg-amber-500/80"
+            style={{ height: `${(count / max) * 100}%`, minHeight: count > 0 ? 2 : 0 }}
+          />
+        ))}
+      </div>
+      <div className="mt-1 flex gap-1">
+        {[1, 2, 3, 4, 5].map((n) => (
+          <div key={n} className="flex-1 text-center text-[10px] text-muted-foreground/40">{n}</div>
+        ))}
+      </div>
+    </div>
+  )
+}