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

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

Author: Seungwoo321