feat(chat): add emoji reaction support for chat message

[mishraditi]

Description

This PR enables support for reacting to chat messages using emojis.

What it does

• Emoji reaction bar appears on message hover
• Quick reactions (👍👎❤️😂😮👋);
• Reactions are displayed below messages with counts
• Clicking your own reaction again removes it

How to test

• Open a meeting link in Chrome
• Open the same link in Incognito (second participant)
• Send a chat message from one participant
• Hover over the message and add a reaction
• Verify the reaction appears for both participants

Screenshot

chat.emoji.reaction.mp4

[jitsi-jenkins]

Hi, thanks for your contribution!
If you haven't already done so, could you please make sure you sign our CLA (https://jitsi.org/icla for individuals and https://jitsi.org/ccla for corporations)? We would unfortunately be unable to merge your patch unless we have that piece :(.