feat: implement elegant slide-in floating control panel for edit mode

Replaced the intrusive blue status bar with a sleek slide-in control panel:

UI/UX Improvements:
- Minimized ribbon (📝 icon) on top-right that slides out to full panel
- Beautiful gradient design with backdrop blur effects
- Smooth CSS transitions with cubic-bezier easing
- Auto-expand on load, then minimize after 2 seconds
- Click outside to close, click ribbon to toggle

Features Combined:
- Status indicators with dynamic icons (⏳ loading, ✅ success, ❌ error)
- Save & Download and Preview buttons in clean grid layout
- Version information in panel header
- Error reporting with expandable details
- Responsive design for mobile devices

Technical Changes:
- Replaced old floating-header with integrated control panel
- Enhanced status update function with visual state management
- Added toggle functionality with click-outside-to-close
- Improved typography and spacing throughout
- Updated test to match new element ID structure

This provides a much cleaner editing experience with better space utilization
while maintaining all previous functionality and adding visual polish.

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

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

tests/test_issue_144_edit_mode_regression.py

@@ -226,7 +226,7 @@ class TestEditModeRegression:
         )
 
         # Should contain error handling elements
-        assert 'id="markitect-status"' in html_content
+        assert 'id="markitect-control-panel"' in html_content
         assert 'id="status-message"' in html_content
         assert 'id="error-details"' in html_content
         assert 'reportEditModeError' in html_content