✨ Add universal chat with tool streaming and auto-scroll support

Make chat modal persist across modes with access to all generated content
(commit messages, PR descriptions, reviews, changelogs, release notes).

Key improvements:
- Stream tool execution status to chat UI as Iris works
- Auto-scroll chat to bottom on new messages, disable when user scrolls up
- Expand chat modal to 80% of screen for better readability
- Improve JSON extraction to handle various LLM response formats
- Add PageUp/PageDown for faster chat navigation
- Fix markdown header rendering for all heading levels
- Improve word wrapping for long lines and long words

Author: hyperb1iss