feat: add emoji/image support, group chat, message retention, and premium tiers

- Add emoji picker with palette icon and quick emoji selection
- Add IMAGE message type with mediaUrl for image sharing
- Add mediaUrl field to Message model
- Add PremiumSeat tier and messageRetentionDays fields
- Fix GET /api/conversations to use userId (not id)
- Fix POST /api/conversations to use create with userId
- Add conversationSchema for group chat validation
- Update messageSchema to include mediaUrl and IMAGE validation
- Add DELETE /api/messages for admin pruning
- Filter expired/deleted messages in GET
- Set expiresAt based on Premium tier on message creation
- Update CHAT_DESIGN.md documentation