fix: Color Picker resolves blend modes when sampling canvas color

[Copilot]

Summary

The color picker was returning raw layer pixel values, ignoring CSS mix-blend-mode applied to raster layer canvases. As shown in the screenshot above, hovering over the visually black multiply-blended intersection returned R:235 G:39 B:39 (the raw red layer color) instead of the composited result.

Root cause: getColorAtCoordinate used Konva's stage.toCanvas(), which renders layers via Canvas 2D API and does not apply CSS mix-blend-mode — a browser compositor-level property set on the HTML canvas elements by CanvasEntityAdapterRasterLayer.syncGlobalCompositeOperation.

Fix (konva/util.ts — getColorAtCoordinate):

• Replace stage.toCanvas() with manual layer-by-layer compositing into a 1×1 offscreen canvas
• Iterate stage.getLayers() in draw order; skip invisible layers (preserves existing background-hide behavior)
• Read htmlCanvas.style.mixBlendMode from each layer's underlying _canvas element (same internal API pattern already used in CanvasEntityAdapterRasterLayer)
• Map normal → source-over; all other CSS blend mode names are identical to Canvas 2D globalCompositeOperation values
• Convert CSS pixel coordinates to physical pixels via window.devicePixelRatio for HiDPI correctness
• Guard drawImage with bounds check on source coordinates

Related Issues / Discussions

QA Instructions

1. Create two raster layers — paint a solid shape on each with different colors
2. Set the upper layer to a non-normal blend mode (e.g. Multiply)
3. Select the Color Picker tool and hover over the blended region
4. Verify the sampled RGBA values match the visually composited color, not the raw layer color

Merge Plan

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• ❗Changes to a redux slice have a corresponding migration
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)