0.99.6

Fix UI: Clear Chat button overlap and add local testing

- Replace ha-button with native button element to fix z-index issues
- Add proper positioning and z-index to header and clear button
- Improve button styling with native CSS properties
- Add local frontend testing environment
  - tests/frontend/test_panel.html for standalone testing
  - tests/frontend/serve.py as simple HTTP server
- Update README with local testing instructions

This fixes the issue where the Clear Chat button was overlapping
with other UI elements in the header.

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

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