feat: implement MoveCommand for undoable layer movement

Critical Fix - Move Tool Undo/Redo:
- Create MoveCommand class to track layer position changes
- Capture initial and final positions during move operation
- Only add to history if position actually changed
- Real-time visual feedback during drag (via updateLayer)
- Single undo point per move operation (not per pixel)

Command Pattern:
- MoveCommand extends BaseCommand
- Implements execute() and undo() methods
- captureAfterPosition() called on pointer up
- hasChanged() prevents no-op entries in history

Files:
- core/commands/move-command.ts - New command class
- tools/move-tool.ts - Updated to use MoveCommand
- core/commands/index.ts - Export MoveCommand

This fixes the critical issue where moving layers had no undo support.
Users can now undo/redo layer movements as expected.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

core/commands/index.ts

@@ -1,3 +1,4 @@
 export * from './base-command';
 export * from './layer-commands';
 export * from './draw-command';
+export * from './move-command';