fix: prevent VuePivottableUi code execution when using VuePivottable

- Remove unnecessary composables export from index.ts
- Add proper default props to VPivottable component
- Fix isolation between VuePivottable and VuePivottableUi components
- Reduce bundle size by removing unused code

Fixes #undefined - TypeError when using VuePivottable without aggregators

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

Co-Authored-By: Claude <[email protected]>

Author: Seungwoo321