- Template Management System: Complete HTML overlay template upload, delete, and real-time editing
- Persistent Template Storage: Cross-platform user template storage (AppData/Library/config)
- Template Priority System: Uploaded templates override built-in templates with same filename
- Custom URL Scheme Handler: overlay:// protocol enables uploaded templates to access overlay.js
- Live Template Switching: Dashboard forms instantly switch to selected template with overlay updates
- Template Code Viewer: View and copy template source code with syntax highlighting
- File Watcher Integration: Automatic template reloading when files change on disk
- Enhanced Template UI: Popup previews, source viewer, dynamic template selection

Fixes:
- API endpoint mismatch: Fixed /api/overlay/update vs /api/overlay URL inconsistency
- Template selection: Fixed video test page template selection with auto .html extension
- JavaScript accessibility: Custom overlay:// scheme allows uploaded templates to access overlay.js
- MessageBuilder: Added missing overlay_update() method
- Template reload logic: Load specific template from form instead of reloading current

Technical details:
- OverlayUrlSchemeHandler for JavaScript file serving via overlay:// protocol
- Qt Resource Collections (QRC) integration for embedded JavaScript files
- PyInstaller compatibility with persistent user data outside executable bundle
- WebChannel communication enhanced between Qt application and JavaScript overlays
- Cross-platform persistent storage paths with proper fallback mechanisms