Fix mobile color picker drag bounds

[tracygardner]

Motivation

• On small screens the color picker couldn't be dragged below the canvas because vertical bounds were clamped to the canvas area height; the goal is to allow vertical dragging into the viewport while keeping horizontal movement constrained to the canvas area.

Description

• Update the pointermove handler in ui/colourpicker.js to compute maxLeft from the parent width and maxTopInViewport from window.innerHeight - parentRect.top - pickerRect.height, and clamp left to [0, maxLeft] and top to [0, maxTopInViewport] instead of using the parent's height.

Testing

• Ran a Playwright script that opens the dev server at a mobile viewport (390x844), clicks #colorPickerButton, and captures a screenshot of .custom-color-picker, which completed successfully and produced artifacts/color-picker-mobile.png.
• Ran npm run build, which completed successfully.
• Ran npx eslint ui/colourpicker.js, which failed due to pre-existing lint issues in the file (unused variables, duplicate class members, and empty block statements) unrelated to this change.

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Codex Task