Custom reactions

### Your use case

#### What would you like to do?

Have the ability to create custom reactions like Discord's soundboard feature.

#### Why would you like to do it?

The current reactions don't suit my use cases.

#### How would you like to achieve it?

Like how sticker/emoji packs are implemented in element;
- Being able to create a reaction pack
- A custom reaction sound is tied to an emoji (sticker pack emojis would be nice, but standard emojis would work fine).
- Using reaction packs in the reaction selector, the same way as from the emoji selector in a text channel in elements.

### Have you considered any alternatives?

Yes, such as creating my own soundboard application that plays the sound through the selected microphone However, this doesn't work well for mobile.

### Additional context

_No response_

Start by reviewing Element’s existing sticker and emoji pack implementation, which the issue identifies as the intended model. Then trace how reaction and emoji selectors are integrated, including mobile behavior. Done means users can create and use reaction packs with emoji-linked sounds from the reaction selector.

Written by the indexing model from the issue text.