feat: enhance control panel UI and resize functionality

Panel UI improvements:
- Replace heading elements (h1-h6) with styled divs to avoid navigation interference
- Change ContentsControl position from northwest to west for better accessibility

Panel collapse/expand enhancements:
- Fix panel dragging to prevent unexpected positioning jumps
- Keep panel width and upper-left position when collapsing to header-only mode
- Complete height reduction when collapsed (no minimal size maintained)
- Toggle resize handle visibility based on panel state

Resize handle improvements:
- Change resize symbol from arrow to clean dot (●) in bottom-right corner
- Remove background circle, show transparent dot only
- Fix resize direction to properly follow mouse movement from bottom-right
- Set dynamic minimum size constraints (header height + padding)
- Allow arbitrary panel sizing with proper bounds checking
- Reset panel size to defaults when closed/collapsed

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

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

testdrive-jsui/static/js/controls/debug-control.js

@@ -432,7 +432,7 @@ class DebugControl extends ControlBase {
             if (content) {
                 content.innerHTML = `
                     <div style="padding: 1rem; font-size: 0.8rem;">
-                        <h4 style="margin-top: 0; margin-bottom: 1rem;">Debug Information</h4>
+                        <div style="margin-top: 0; margin-bottom: 1rem; font-weight: 600; font-size: 1.1em; color: #333;">Debug Information</div>
 
                         ${this.generateSystemInfoHTML()}
                         ${this.generatePerformanceHTML()}