feat: Add SVG engine support for draggable node helper lines

[UmbraCi]

Description

This PR enhances the draggable node plugin to use SVG for drawing helper lines when the SVG engine is enabled, improving performance for large-scale mind maps.

Changes

Modified Files

• src/plugins/jsmind.draggable-node.js

Key Features

1. Engine Detection

   • Detects the current rendering engine (SVG or Canvas) in the constructor
   • Stores the engine type in is_svg_engine property

2. SVG Support

   • Creates SVG element instead of Canvas when SVG engine is used
   • Draws helper lines using SVG path elements with bezier curves
   • Reuses the bezier curve logic from SvgGraph for consistency

3. Backward Compatibility

   • Canvas engine continues to work as before
   • No breaking changes to existing code
   • Fully compatible with all existing features

Implementation Details

New Methods

• _create_svg_element(tag): Creates SVG elements with proper namespace
• svg_draw_line(x1, y1, x2, y2, color): Draws helper lines using SVG
• _svg_bezier_to(path, x1, y1, x2, y2): Draws bezier curves (reuses SvgGraph logic)

Modified Methods

• create_canvas(): Creates SVG or Canvas based on engine type

esize(): Handles sizing for both SVG and Canvas

• magnet_shadow(): Routes to appropriate drawing method based on engine
• clear_lines(): Clears SVG or Canvas based on engine type

Performance Benefits

SVG Engine:

• Incremental updates: Only modifies the helper line element
• Lower memory usage: Stores only one SVG element instead of canvas buffer
• Visual consistency: Uses the same rendering engine as mind map connections
• Better interactivity: SVG elements can directly bind events

Canvas Engine:

• Maintains original behavior
• No performance impact
• Fully backward compatible

Code Reuse

• Reuses SVG element creation method from SvgGraph
• Reuses bezier curve drawing algorithm from SvgGraph
• Maintains consistent code style with existing SVG implementation

Testing

• Tested with both SVG and Canvas engines
• Verified helper lines display correctly during drag operations
• Confirmed backward compatibility with Canvas engine
• No IDE diagnostics errors

Related Issues

Improves performance for large-scale mind maps (500+ nodes) when using SVG engine.

Checklist

• [x] Code follows the project style guidelines
• [x] Changes are backward compatible
• [x] No breaking changes
• [x] Tested with both SVG and Canvas engines
• [x] Documentation provided