Implement pencil drawing functionality

[Copilot]

The setupDrawing() function in imgkit/features/image_layer_events.js was a stub with only TODO comments, breaking the pencil drawing feature.

Changes

• Implemented canvas drawing in setupDrawing()

  • Mouse event handlers (mousedown/move/up/leave) for path drawing
  • Canvas API calls: beginPath(), moveTo(), lineTo(), stroke()
  • Round line caps/joins for smooth strokes
  • Line width initialization with fallback to 1px

• History integration

  • Converts canvas to data URL → buffer on mouseup/mouseleave
  • Calls updatePreview() to persist drawing state for undo/redo

Usage

// Drawing flow already wired in main_renderer.js:
// 1. Click draw button → sets ImageMode.DRAWING
// 2. Pencil cursor appears (image_mode.js)
// 3. setupDrawing() handlers now active
// 4. Ctrl+Scroll adjusts line width (existing code)

The implementation follows the existing pattern from setupCropping() and setupMagnifying().

Original prompt

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This section details on the original issue you should resolve

<issue_title>enable pencil drawing</issue_title>
<issue_description>function is broken, restore is a mission</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes enable pencil drawing #119

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