Add reactions to text messages

## Feature
Add support for reactions on text messages in chat.

## Motivation
Participants want a lightweight way to respond to messages without adding a separate text reply.

## Proposed behavior
- Allow a user to add an emoji reaction to a text message.
- Display reactions and their counts on the message.
- Allow users to see that a message has reactions.

## Acceptance criteria
- Users can add a reaction to a text message.
- Reactions are visibly associated with the relevant message.
- Multiple reactions and reaction counts render correctly.

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Requested by **Ishaan** during a Conclave meeting.

No files or tests are identified in the issue. Start by locating the text-message handling in the web and mobile clients and the related message data path, then trace how reactions can be stored and rendered. Done means users can add emoji reactions, see them attached to the correct message, and see multiple reactions with accurate counts.

Written by the indexing model from the issue text.