feat(gif): recently-used GIFs in the picker

The GIF picker was a bare Giphy search grid with no memory of what you've
sent, so re-sending a go-to reaction GIF meant re-typing the search every
time. Add a "Recent" row at the top of the picker (default view; hidden
while searching) for one-click re-sending.

- New persisted state state/recentGifs.ts: recentGifsAtom (localStorage,
  cinny_recent_gifs_v1, getOnInit) + pure addRecentGif (dedupe-by-url
  move-to-front, cap 16, ignore empty url), with 5 unit tests.
- GifPicker records every sent GIF (from search or the Recent row) to the
  front, and renders a 3-up thumbnail grid of recents above the search
  grid when there are recents and no active search term. Section label
  matches the picker's existing `// GIF_SEARCH` treatment (lotusTerminal)
  or a muted label otherwise.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/state/recentGifs.ts

@@ -0,0 +1,48 @@
+import { atom } from 'jotai';
+import { atomWithStorage, createJSONStorage } from 'jotai/utils';
+
+export type RecentGif = {
+  url: string;
+  width: number;
+  height: number;
+};
+
+const STORAGE_KEY = 'cinny_recent_gifs_v1';
+const MAX_RECENT_GIFS = 16;
+
+// getOnInit reads localStorage synchronously so the Recent row is present on the
+// first render of the GIF picker (no flash of the empty default).
+const internalAtom = atomWithStorage<RecentGif[]>(
+  STORAGE_KEY,
+  [],
+  createJSONStorage(() => localStorage),
+  { getOnInit: true },
+);
+
+/**
+ * Global atom: the most recently sent GIFs, newest first, deduped by url, capped
+ * at MAX_RECENT_GIFS. Backed by localStorage (device-local convenience).
+ */
+export const recentGifsAtom = atom(
+  (get): RecentGif[] => get(internalAtom),
+  (_get, set, updater: RecentGif[] | ((prev: RecentGif[]) => RecentGif[])) => {
+    set(internalAtom, (prev) => {
+      const prevList = Array.isArray(prev) ? prev : [];
+      return typeof updater === 'function' ? updater(prevList) : updater;
+    });
+  },
+);
+
+/**
+ * Prepend a GIF: ignores an empty url, de-dupes by url (moving an existing entry
+ * to the front), and caps the list at `max`. Pure — returns a new array.
+ */
+export const addRecentGif = (
+  prev: RecentGif[],
+  gif: RecentGif,
+  max = MAX_RECENT_GIFS,
+): RecentGif[] => {
+  if (!gif.url) return prev;
+  const withoutDupe = prev.filter((g) => g.url !== gif.url);
+  return [gif, ...withoutDupe].slice(0, max);
+};