Make colour picker commit on blur and add draggable popup

[tracygardner]

Motivation

• Keep the currently selected color if the user moves focus away from the colour picker (e.g., clicking outside or on the backdrop) so a selection isn't lost when the popup closes.
• Let users reposition the colour picker by dragging so it doesn't obscure important canvas content.

Description

• Added a drag handle element in the picker template and styles in ui/colourpicker.css to render a visible grip and set cursor: move and appropriate touch behavior.
• Implemented pointer-based dragging in ui/colourpicker.js via a new makePopupDraggable() method that constrains the popup position to the canvas parent and sets this.userMovedPicker to preserve manual placement on reopen.
• Centralised close behavior through a new close(options) signature with flags commitColor and triggerOnClose, and updated backdrop/outside-click handlers and confirmColor() to use these flags so outside-close commits the selected color.
• Updated open() to skip automatic repositioning if the user has manually moved the picker, preserving the user-chosen location.

Testing

• Built the app with npm run build, which completed successfully (build warnings about large chunks were reported but the build succeeded).
• Launched a dev server with npm run dev and performed manual verification of the picker UI and drag behavior.
• Ran a Playwright script that opened the app, opened the colour picker, dragged the popup via the new .color-picker-drag-handle, and captured a screenshot artifact showing the draggable popup (script executed and produced the artifact).

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Codex Task