Add iSamples Interactive Explorer tutorial

Quarto version of the Jupyter notebook explorer with:
- Cesium 3D globe with source-colored points
- DuckDB-WASM for in-browser parquet queries
- Observable Inputs for source filter and sample count
- Interactive data table with fly-to on selection
- Sample cards matching Jupyter version design
- Panel tabset UI (Globe / Data Table / Sample Card / Statistics)

Companion to isamples-python/examples/basic/isamples_explorer.ipynb

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: rdhyee