Add zap counts on notes (Phase 1 #2)

- fetchZapCount(eventId): fetches kind 9735 receipts for an event,
  parses millisat amounts from embedded zap request description tags,
  returns { count, totalSats }
- useZapCount hook: session-cached, same pattern as useReactionCount
- NoteCard: zap button shows "⚡ N sats" when total > 0, falls back
  to "⚡ zap" when no zaps yet; stats row shown for logged-out users
  displaying ♥ and ⚡ counts when non-zero

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/hooks/useZapCount.ts

@@ -0,0 +1,23 @@
+import { useEffect, useState } from "react";
+import { fetchZapCount } from "../lib/nostr";
+
+interface ZapData { count: number; totalSats: number; }
+
+const cache = new Map<string, ZapData>();
+
+export function useZapCount(eventId: string): ZapData | null {
+  const [data, setData] = useState<ZapData | null>(() => cache.get(eventId) ?? null);
+
+  useEffect(() => {
+    if (cache.has(eventId)) {
+      setData(cache.get(eventId)!);
+      return;
+    }
+    fetchZapCount(eventId).then((d) => {
+      cache.set(eventId, d);
+      setData(d);
+    });
+  }, [eventId]);
+
+  return data;
+}