add file browser component and widget

examples/widget_example/README.md

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+# FileBrowser Widget
+
+A WebAssembly-based file browser widget that can be embedded in any web application.
+
+## Features
+
+- File and directory browsing
+- File upload with progress tracking (using TUS protocol)
+- File download
+- Directory creation and deletion
+- File editing (markdown with live preview, text files)
+
+## Running the Example
+
+1. **Start a local server** (required for WASM):
+   ```bash
+   python3 -m http.server 8081
+   # or
+   npx serve .
+   ```
+
+2. **Start the mock backend** (in another terminal):
+   ```bash
+   cd ../file_browser_demo
+   cargo run --bin mock_server
+   ```
+
+3. **Open the example**:
+   - Navigate to `http://localhost:8081`
+   - The widget will load with a configuration panel
+   - Try different settings and see them applied in real-time
+
+## Key Features Demonstrated
+
+### Runtime Configuration
+The example shows how to configure the widget at runtime without rebuilding:
+
+```javascript
+// Create base configuration
+const config = create_default_config('http://localhost:3001/files');
+
+// Apply runtime settings using corrected method names
+config.setTheme('light');                    // Theme selection
+config.setMaxFileSize(100 * 1024 * 1024);   // 100MB limit
+config.setShowUpload(true);                  // Enable upload
+config.setShowDownload(true);                // Enable download
+config.setShowDelete(false);                 // Disable delete
+config.setInitialPath('documents/');         // Start in documents folder
+
+// Create widget with configuration
+const widget = create_file_browser_widget('container-id', config);
+```
+
+### Dynamic Reconfiguration
+The widget can be recreated with new settings:
+
+```javascript
+function updateWidget() {
+    // Destroy existing widget
+    if (currentWidget) {
+        currentWidget.destroy();
+    }
+    
+    // Create new widget with updated config
+    const newConfig = create_default_config(newEndpoint);
+    newConfig.setTheme(selectedTheme);
+    currentWidget = create_file_browser_widget('container', newConfig);
+}
+```
+
+### Error Handling
+The example includes comprehensive error handling:
+
+- WASM initialization errors
+- Browser compatibility checks
+- Widget creation failures
+- Network connectivity issues
+
+## Widget API Reference
+
+### Core Functions
+
+```javascript
+// Initialize WASM module (call once)
+await init();
+
+// Create default configuration
+const config = create_default_config(baseEndpoint);
+
+// Create widget instance
+const widget = create_file_browser_widget(containerId, config);
+
+// Utility functions
+const version = get_version();
+const isCompatible = check_browser_compatibility();
+```
+
+### Configuration Methods
+
+```javascript
+config.setTheme(theme);              // 'light' | 'dark'
+config.setMaxFileSize(bytes);        // Number in bytes
+config.setShowUpload(enabled);       // Boolean
+config.setShowDownload(enabled);     // Boolean
+config.setShowDelete(enabled);       // Boolean
+config.setCssClasses(classes);       // String of CSS classes
+config.setInitialPath(path);         // String path
+```
+
+### Widget Handle Methods
+
+```javascript
+widget.destroy();                    // Clean up widget
+// Note: Currently no update method - recreate widget for config changes
+```
+
+## Advanced Usage
+
+### Custom Styling
+```javascript
+config.setCssClasses('my-custom-theme dark-mode');
+```
+
+### Multiple Widgets
+```javascript
+const widget1 = create_file_browser_widget('container1', config1);
+const widget2 = create_file_browser_widget('container2', config2);
+```
+
+### Integration with Frameworks
+
+**React:**
+```jsx
+function FileBrowserComponent({ endpoint }) {
+    const containerRef = useRef();
+    const widgetRef = useRef();
+    
+    useEffect(() => {
+        async function initWidget() {
+            await init();
+            const config = create_default_config(endpoint);
+            widgetRef.current = create_file_browser_widget(
+                containerRef.current, 
+                config
+            );
+        }
+        initWidget();
+        
+        return () => widgetRef.current?.destroy();
+    }, [endpoint]);
+    
+    return <div ref={containerRef} />;
+}
+```
+
+**Vue:**
+```vue
+<template>
+  <div ref="container"></div>
+</template>
+
+<script>
+export default {
+  async mounted() {
+    await init();
+    const config = create_default_config(this.endpoint);
+    this.widget = create_file_browser_widget(this.$refs.container, config);
+  },
+  beforeUnmount() {
+    this.widget?.destroy();
+  }
+}
+</script>
+```
+
+## Troubleshooting
+
+### Common Issues
+
+1. **"config.setTheme is not a function"**
+   - Ensure you're using the latest widget build
+   - Check that WASM module is properly initialized
+
+2. **Widget not appearing**
+   - Verify container element exists
+   - Check browser console for errors
+   - Ensure WASM files are served correctly
+
+3. **Backend connection errors**
+   - Verify backend is running on specified endpoint
+   - Check CORS configuration
+   - Ensure all required API endpoints are implemented
+
+### Debug Mode
+```javascript
+// Enable debug logging
+console.log('Widget version:', get_version());
+console.log('Browser compatible:', check_browser_compatibility());
+```
+
+## Performance Notes
+
+- **Initial Load**: ~368KB total (WASM + JS)
+- **Runtime Memory**: ~2-5MB depending on file list size
+- **Startup Time**: ~100-300ms on modern browsers
+- **File Operations**: Near-native performance via WASM
+
+The widget is optimized for production use with minimal overhead.