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/status-control.js

@@ -132,7 +132,7 @@ class StatusControl extends ControlBase {
 
             return `
                 <div style="padding: 1rem; font-size: 0.8rem;">
-                    <h4 style="margin-top: 0; margin-bottom: 1rem;">Document Statistics</h4>
+                    <div style="margin-top: 0; margin-bottom: 1rem; font-weight: 600; font-size: 1.1em; color: #333;">Document Statistics</div>
 
                     <div class="stats-grid" style="display: grid; grid-template-columns: 1fr 1fr; gap: 0.5rem; margin-bottom: 1rem;">
                         <div class="stat-item">
@@ -161,7 +161,7 @@ class StatusControl extends ControlBase {
                     </div>
 
                     <div class="structure-stats" style="border-top: 1px solid #eee; padding-top: 0.5rem; margin-bottom: 1rem;">
-                        <h5 style="margin: 0 0 0.5rem 0; font-size: 0.9em;">Document Structure</h5>
+                        <div style="margin: 0 0 0.5rem 0; font-size: 0.9em; font-weight: 600; color: #555;">Document Structure</div>
 
                         <div style="display: flex; justify-content: space-between; margin-bottom: 0.3rem;">
                             <span>Paragraphs:</span>