⚡ Optimize IconEditor preview rendering and fix line thickness logic

[google-labs-jules]

💡 What:

• Implemented PreviewPainter as a CustomPainter to draw Line and Rectangle previews directly on the canvas.
• Modified IconEditor to use PreviewPainter instead of _previewLine and _previewRectangle which were modifying a copy of the image buffer.
• Fixed line thickness logic in ImageUtils.drawLine (and PreviewPainter) to use thickness ~/ 2 for correct width scaling.
• Fixed rectangle drawing in PreviewPainter to avoid overdrawing corners (important for semi-transparent colors).

🎯 Why:

• Performance: Previously, dragging a line or rectangle caused a full copy of the img.Image buffer (which can be large) on every frame. This caused lag on slower devices or large images. Using a CustomPainter is extremely lightweight.
• Correctness: The line thickness logic was producing lines almost double the requested thickness (e.g., size 3 -> width 5). This is now corrected to size 3 -> width 3.

📊 Measured Improvement:

• Baseline: Every drag event triggered img.Image.from(image) and ImageUtils.drawLine/Rect (pixel manipulation).
• Optimization: Drag events now only trigger a setState and a lightweight Canvas draw.
• Impact: While specific benchmarks could not be run due to environment limitations, the complexity reduction from O(ImagePixels) per frame to O(ShapePixels) per frame is a massive theoretical improvement (e.g. from copying 65k pixels to drawing 100 pixels).

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PR created automatically by Jules for task 15277165099272657853 started by @ArinFaraj

[google-labs-jules]

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