Description

This PR implements the mention autocomplete feature for the Android version of bitchat, bringing it to parity with the iOS version. Users can now type "@" followed by a few characters to see a list of connected users they can mention in their messages.

Features Added

Mention Autocomplete Suggestions

When typing "@" in the chat input, a suggestions box appears showing connected users
Suggestions are filtered based on the typed characters (e.g., "@jo" shows users whose nicknames start with "jo")
Clicking on a suggestion inserts the mention at the current cursor position
Visual Mentions Highlighting

Mentions are visually highlighted in orange in the chat input field
Mentions in received messages are also highlighted consistently with iOS styling
Both slash commands and mentions can be styled simultaneously
Context-Aware Mention Insertion

Mentions are inserted at the cursor position without replacing existing text
Works correctly even when typing in the middle of a sentence

Checklist

• I have read the contribution guidelines: https://github.com/permissionlesstech/bitchat-android?tab=readme-ov-file#contributing
• I have performed a self-review of my code

• I have mentioned the corresponding issue and the relevant keyword (e.g., "Closes: #xy") in the description (see https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue)
• If it is a core feature, I have added automated tests