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>

LOTUS_FEATURES.md

@@ -674,6 +674,7 @@ The indicator is hidden once the server confirms the event (when the internal st
 - Giphy-powered picker accessible from the composer toolbar
 - The button is only shown when `gifApiKey` is set in `config.json`
 - Selected GIFs are sent as `m.image` events
+- **Recently used**: a "Recent" row at the top of the picker (shown on the default view, hidden while searching) surfaces the GIFs you last sent for one-click re-sending — no re-searching. Persisted in localStorage (`cinny_recent_gifs_v1`), deduped by url, most-recent-first, capped at 16, via the pure/unit-tested `addRecentGif` (`src/app/state/recentGifs.ts`).
 - Picker UI is styled with TDS variables when the TDS theme is active
 - Located at `src/app/components/GifPicker.tsx`