enh: add username option and align messages accordingly.

[shsiddhant]

Pull Request Summary

Adds option to choose username from participants during chat import and aligns messages according to username.

Type of Change

Choose one or more:

• Bugfix
• New Feature

Changes

1. Set minimum size and initial geometry for main window.
2. Add participants property and a modifiable username attribute to Conversation.
3. Add option to choose username from participants during chat import.
4. Fix messages alignment according to username - on the right if sender is same as username, on the left otherwise.

Checklist

• Code runs without errors
• Code style and lint checks passed