🤖 GitHub-like hover buttons for code review comments (#326)

## Changes

Transforms code review commenting to GitHub-like interaction pattern:

### UX Flow
- **Hover line** → `+` button fades in
- **Click +** → Opens comment textarea
- **Click line content** → Normal text selection works ✨
- **Enter** → Submit review note
- **Shift+Enter** → Newline
- **Esc** → Cancel

### Key Improvements

1. **Hover-revealed comment buttons**
   - Hidden by default, appear on hover
   - Only buttons trigger comment mode
   - Text selection works everywhere else

2. **Improved textarea UX**
   - Starts at 3 lines, auto-expands with content
   - Less padding (seamless background)
   - Consistent shortcuts with chat (Enter to submit)

3. **Performance optimization**
   - Extracted textarea to separate memoized component
   - No re-renders of diff lines while typing

4. **Infrastructure**
   - Enabled @emotion/babel-plugin for component selectors
   - Consolidated duplicate vite config (DRY)
   - Use Tooltip component instead of title attribute

## Testing

- ✅ TypeScript compilation passes
- ✅ Text selection works on diff lines
- ✅ Typing is instant (no re-render lag)
- ✅ All keyboard shortcuts work

_Generated with `cmux`_

Author: ammar-agent