Accessibility improvements for bottom drawer and sidebar overlay

## Bottom drawer
- Add focus trap when expanded (currently focus can escape behind the drawer)
- Add \`aria-expanded\` attribute to the handle button
- Consider adding \`aria-label=\"Toggle sidebar drawer\"\` to the handle button

## Sidebar overlay
- Add focus trap when open
- Add \`role=\"dialog\"\` and \`aria-modal=\"true\"\` to the panel
- Auto-focus the close button when opening
- Escape key dismiss is already implemented

## Split pane header
- Version dropdown and 3-dots menu should have \`aria-label\` attributes
- The floating header should be announced to screen readers

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