feat(frontend): in-browser GeoParquet preview via DuckDB WASM

## Problem

Users upload a file, wait for the full ingestion pipeline (scan → convert → register), then discover the data is wrong (bad CRS, wrong columns, unexpected nulls). Feedback loop is minutes.

## Proposal

Before committing to ingestion, offer an instant in-browser preview using DuckDB WASM (as [Geospatial Atlas](https://github.com/do-me/geospatial-atlas) does in its frontend-mode). User drops a parquet/GeoParquet, sees a sample on a map + tabular summary immediately, then decides whether to ingest.

## Scope

- New \"Preview\" entry point on the upload UI
- DuckDB WASM reads the file client-side
- Show: bounding box on map, row count, column schema, first N rows, geometry type
- \"Proceed with ingestion\" button continues the normal flow
- Initially GeoParquet/parquet only; later extend to small GeoJSON

## Out of scope

- COG/raster preview (different tech)
- Full filtering/cross-filter UX (that's issue for WebGPU layer)

## References

- https://github.com/do-me/geospatial-atlas frontend-mode