Add message navigation, focus indicator, and expand/collapse

- Page Up/Down navigates between messages instead of scrolling lines
- Focus indicator (▎) shows which message is selected in preview
- Space bar toggles expand/collapse of focused message
- Mouse click selects message, double-click expands/collapses
- Status bar shows dynamic "expand"/"collapse" label
- Multiple messages can be expanded independently
- Added 17 unit tests for new navigation/expansion features

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: zippoxer