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.test.ts

@@ -0,0 +1,56 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+
+// The module evaluates atomWithStorage(..., { getOnInit: true }), which reads
+// localStorage at load time. node has none, so install a no-op mock, then import
+// dynamically (a static import would hoist above the mock and evaluate too early).
+(globalThis as { localStorage?: unknown }).localStorage = {
+  getItem: () => null,
+  setItem: () => undefined,
+  removeItem: () => undefined,
+};
+
+const { addRecentGif } = await import('./recentGifs');
+
+const gif = (url: string, width = 100, height = 100) => ({ url, width, height });
+
+test('addRecentGif prepends a new gif', () => {
+  const out = addRecentGif([gif('a'), gif('b')], gif('c'));
+  assert.deepEqual(
+    out.map((g) => g.url),
+    ['c', 'a', 'b'],
+  );
+});
+
+test('addRecentGif de-dupes by url, moving the entry to the front', () => {
+  const out = addRecentGif([gif('a'), gif('b'), gif('c')], gif('b'));
+  assert.deepEqual(
+    out.map((g) => g.url),
+    ['b', 'a', 'c'],
+  );
+});
+
+test('addRecentGif ignores an empty url', () => {
+  assert.deepEqual(addRecentGif([gif('a')], gif('')), [gif('a')]);
+});
+
+test('addRecentGif caps the list at 16, dropping the oldest', () => {
+  const sixteen = Array.from({ length: 16 }, (_, i) => gif(`u${i}`));
+  const out = addRecentGif(sixteen, gif('new'));
+  assert.equal(out.length, 16);
+  assert.equal(out[0].url, 'new');
+  assert.equal(
+    out.some((g) => g.url === 'u15'),
+    false,
+  );
+});
+
+test('addRecentGif does not mutate its input', () => {
+  const input = [gif('a'), gif('b')];
+  const before = input.map((g) => g.url);
+  addRecentGif(input, gif('c'));
+  assert.deepEqual(
+    input.map((g) => g.url),
+    before,
+  );
+});