Add copy and paste controls for splat transforms

Problem

When working with multiple splats, there is currently no quick way to transfer transform values from one splat layer to another.

This makes workflows such as aligning related splats, positioning replacement assets, duplicating placement, and comparing edited variants unnecessarily manual. Users must copy individual position, rotation, and scale values by hand, which is repetitive, slow and error-prone.

Proposed solution

Add copy and paste controls to the Transform panel.

The copy action should allow users to choose which transform components to capture:

• Position
• Rotation
• Position and rotation
• All transforms, including scale

After selecting another splat, Paste should apply only the copied components while preserving the destination splat's other transform values.

Expected behavior

• Copy is available when a splat is selected.
• Paste becomes available after transform values have been copied and another splat is selected.
• Copied values remain available for repeated pasting across multiple splats.
• Copying again replaces the previously stored transform values.
• Paste is recorded as a single undoable and redoable edit.
• The controls follow existing Transform panel UI and localization patterns.

Use cases

• Align related splats that share a coordinate frame.

• Transfer placement between original and edited asset variants.

• Apply the same position or rotation to several splats.

• Replace a splat while preserving its scene placement.

• Avoid manually reproducing transform values.

  An implementation is available in PR #1000. https://github.com/playcanvas/supersplat/pull/1000