Refactor controls bar background styling with platform-specific effects:
- Add Liquid Glass effect for iOS 26+ using new glassEffect API
- Fallback to ultraThinMaterial for iOS 15-25
- Fallback to blurred black overlay for iOS 14 and earlier
- Extract background logic into reusable applyControlsBackground modifier
- Adjust controls bar vertical offset for better alignment
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>
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