Emoji picker, stickers, GIF, and media UX

## Summary
Expand message-expression UX around emoji, stickers, GIFs, and media selection with Discord-like behavior on web desktop first.

## Scope
- Deliver a first-class emoji picker with recents, search, and custom/unicode integration.
- Add sticker and GIF insertion workflows appropriate for a self-hosted deployment.
- Improve expression/media selection UX in the composer and reaction flows.
- Keep permissions and availability clear for custom media types.

## Acceptance Criteria
- [ ] Emoji selection is fast, searchable, and works across unicode and custom emoji.
- [ ] Sticker and GIF workflows feel first-class rather than ad hoc file uploads.
- [ ] Recent/frequent expression UX persists in a user-meaningful way.
- [ ] Expression/media surfaces respect permissions and asset availability.

## Operational Notes
- GIF/sticker sourcing must be compatible with self-hosted operator expectations around privacy and third-party dependencies.
- This issue should focus on product UX, not generic attachment plumbing.