Fix eraser tool to properly erase content instead of painting white in dark mode

[Copilot]

Problem

The eraser tool in the Advanced Whiteboard was not functioning correctly in dark mode. Instead of removing drawn content, it was painting white lines over it, which became highly visible against the dark background. This made the eraser behave more like a white pen than an actual eraser.

Root Cause

The eraser implementation used ctx.strokeStyle = "#FFFFFF" to "erase" by painting white over content. While this approach worked in light mode (white background), it failed in dark mode where white lines became visible against the black background.

Solution

Replaced the color-based erasing with proper canvas erasing using globalCompositeOperation = 'destination-out'. This composite operation actually removes pixels from the canvas regardless of background color.

Changes Made

1. Eraser tool logic - Changed from ctx.strokeStyle = "#FFFFFF" to ctx.globalCompositeOperation = 'destination-out'
2. Tool switching - Updated tool selection handler to properly set composite operations when switching between tools
3. Drawing tools - Ensured all other drawing tools explicitly reset to ctx.globalCompositeOperation = 'source-over'

Visual Demonstration

Before (Light Mode): Works correctly

Problem (Dark Mode): White eraser line is visible instead of erasing

After Fix: The eraser now properly removes content in both light and dark modes without leaving visible marks.

Testing

• ✅ Verified eraser removes content without visible traces in both modes
• ✅ Confirmed pen, highlighter, and shape tools continue working correctly
• ✅ Tested tool switching preserves correct composite operations
• ✅ Django project builds and runs without errors

Fixes #613.

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