feat(frontend): metadata search and nearest-neighbor lookup on point layers

## Problem

Once a point dataset is loaded, there's no way to find specific features by name/attribute or locate neighbors of a clicked point.

## Proposal

Borrow the real-time search + nearest-neighbor UX from [Geospatial Atlas](https://github.com/do-me/geospatial-atlas). Free-text search over an indexed column; click a point to see k-nearest neighbors highlighted.

## Scope

- Search bar on the map view for point layers with a searchable text column
- User picks which column is indexed (similar UX to category label editing on categorical rasters)
- Index built client-side via DuckDB WASM for small/medium datasets; server-side full-text via Postgres for large ones
- Click-to-find-neighbors using spatial distance (KNN)

## Dependencies

Pairs well with #216 and #219. Client-side index reuses the DuckDB WASM plumbing from #217.

## References

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

Start with the map view and review the DuckDB WASM plumbing from #217, then compare the proposed search and nearest-neighbor UX with Geospatial Atlas. Clarify how the searchable column is selected and how client-side versus Postgres indexing is chosen. Done means point-layer text search works and clicking a point highlights its k-nearest neighbors.

Written by the indexing model from the issue text.