Indicate active dragging in cursor overlay

[enaboapps]

Summary

• Add a drag cursor overlay event and active-drag state.
• Keep the drag indicator persistent while the mouse button is held, including movement during drag.
• Render a subtle center-dot drag marker in both the Electron fallback overlay and native Windows overlay helper.
• Clear the drag indicator on drag end and drag cleanup.

Closes #198.

Validation

• npm test -- src/main/input/command-executor.test.ts src/main/cursor-overlay-helper-client.test.ts
• npm run build
• npm run native:build