Add network reaction counts to note cards

- fetchReactionCount (kind 7 #e filter) in nostr lib
- useReactionCount hook with module-level cache to avoid refetching
- NoteCard shows count next to like button; increments optimistically on like
- Falls back to "like"/"liked" text when count is zero or still loading

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

src/hooks/useReactionCount.ts

@@ -0,0 +1,29 @@
+import { useEffect, useState } from "react";
+import { fetchReactionCount } from "../lib/nostr";
+
+const cache = new Map<string, number>();
+
+export function useReactionCount(eventId: string): [number | null, (delta: number) => void] {
+  const [count, setCount] = useState<number | null>(() => cache.get(eventId) ?? null);
+
+  useEffect(() => {
+    if (cache.has(eventId)) {
+      setCount(cache.get(eventId)!);
+      return;
+    }
+    fetchReactionCount(eventId).then((n) => {
+      cache.set(eventId, n);
+      setCount(n);
+    });
+  }, [eventId]);
+
+  const adjust = (delta: number) => {
+    setCount((prev) => {
+      const next = (prev ?? 0) + delta;
+      cache.set(eventId, next);
+      return next;
+    });
+  };
+
+  return [count, adjust];
+}