Implement standard rendering and interaction for checkbox and radio button controls

[Copilot]

This PR implements comprehensive visual rendering and interaction support for <input type="checkbox"> and <input type="radio"> elements in JSAR Runtime, addressing the lack of visual representation for these essential form controls.

Problem

Previously, JSAR Runtime only set width and height for checkbox/radio inputs without any visual rendering logic, resulting in invisible controls that couldn't display proper states or provide user feedback. This made forms unusable in spatial web applications.

Solution

Visual Rendering

• Checkbox appearance: Square controls with rounded corners, proper borders, and backgrounds
• Radio button appearance: Circular controls with clean borders and backgrounds
• State visualization:
  • Checkmarks for checked checkboxes
  • Filled dots for selected radio buttons
  • Horizontal bars for indeterminate checkboxes
  • Grayed-out styling for disabled states
  • Blue focus outlines for accessibility

Interaction Support

• Click handling: Toggle checkbox states and set radio button selections
• Focus management: Proper focus/blur with visual feedback
• Disabled state: Complete interaction prevention when disabled
• State persistence: Reliable checked/unchecked/indeterminate state management

DOM API Compatibility

• Standard properties: type, checked, disabled, required, value, name
• Checkbox-specific: indeterminate property support
• Focus methods: focus() and blur() with visual updates
• Form validation: Required checkbox/radio validation

Technical Implementation

The implementation extends the existing layout and rendering infrastructure:

1. Enhanced LayoutHTMLInput: Custom WebContent rendering for checkbox/radio controls using Skia canvas
2. HTMLInputElement: Added interaction handling, focus state management, and indeterminate support
3. DOM Bindings: Full JavaScript API compatibility including indeterminate property
4. ECS Integration: Proper entity/component lifecycle with efficient rendering updates

Example Usage

<!-- Checkbox with all states -->
<input type="checkbox" id="notifications" checked>
<input type="checkbox" id="partial" indeterminate>
<input type="checkbox" id="disabled" disabled>

<!-- Radio button group -->
<input type="radio" name="theme" value="light" checked>
<input type="radio" name="theme" value="dark">

// Programmatic control
const checkbox = document.getElementById('notifications');
checkbox.indeterminate = true;
checkbox.addEventListener('change', e => {
    console.log('Checked:', e.target.checked);
});

Testing & Validation

• Unit tests: Comprehensive test coverage for all functionality
• Interactive demos: Two HTML demo files showcasing all features and states
• Visual consistency: Browser-like appearance with standard color schemes
• Accessibility: Focus outlines and proper disabled state handling

The implementation provides pixel-perfect rendering that matches mainstream browser behavior while integrating seamlessly with JSAR's spatial web capabilities.

Fixes #306.

Warning

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