[Feature] Purchasable custom in-game emojis + gifting effects

## Overview
Purchasable in-game emojis usable during matches. Players can also gift effects to other players. Novelty drives high willingness to pay.

## Scope
- Emoji selection: purchasable beyond default set
- Premium emojis visible to all players in match when used
- Gift mechanic: player can send effect to another player in-match
- Example: purchasable turd nuke emoji

## Acceptance criteria
- [ ] Default emoji set available to all
- [ ] Premium emojis purchasable in shop
- [ ] Emoji displayed to all players when used in match
- [ ] Gift mechanic implemented — player can gift effect to another player
- [ ] Gifted effects notify recipient

## Dependencies
- **Depends on:** OAuth