Add React frontend and clean up legacy HTTP backend

frontend/src/lib/gameRules.ts

@@ -0,0 +1,14 @@
+import type { CardColor, Hand } from '../types';
+
+export function computePlayable(hand: Hand, ledColor: CardColor | null): Set<string> {
+  const keys = Object.keys(hand);
+  if (!ledColor) return new Set(keys);
+
+  const ledKeys = keys.filter((k) => hand[k].color === ledColor);
+  if (ledKeys.length > 0) return new Set(ledKeys);
+
+  const heartKeys = keys.filter((k) => hand[k].color === 'HEARTS');
+  if (heartKeys.length > 0) return new Set(heartKeys);
+
+  return new Set(keys);
+}