Remove unused useReactionCount, seed reactions from batch engagement

- Delete useReactionCount hook (replaced by useReactions)
- Remove fetchReactionCount (no longer referenced)
- Seed per-note reaction cache from fetchBatchEngagement in trending
  feed, so emoji pills render instantly without individual fetches

src/hooks/useReactionCount.ts

@@ -1,29 +0,0 @@
-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];
-}