feat: map viewer at /maps with STAC-backed display metadata

[turban]

Summary

• Adds a display block (colormap, range, nodata) to each built-in dataset template YAML
• Surfaces display metadata through the STAC Render extension on every published STAC collection, alongside climate_api:variable and climate_api:units vendor fields
• Adds GET /maps — a single-page HTML map viewer that reads the STAC catalog, loads per-dataset render and datacube metadata, and renders data via MapLibre GL + @carbonplan/zarr-layer with a time slider derived from cube:dimensions
• Adds a Map viewer link to the landing page

How it works

dataset YAML (display block)
  → STAC collection (renders + stac_extensions)
    → /maps viewer (reads catalog.json, loads collection, init ZarrLayer)

The browser reads the Zarr store directly via the existing /zarr/{dataset_id} HTTP range endpoint. No tile server, no build step.

Display defaults

Dataset	Colormap	Range
CHIRPS precipitation	blues	0–20 mm
ERA5 temperature	rdbu_r	15–40 °C
ERA5 precipitation	blues	0–5 mm/h
WorldPop population	viridis	0–1000 people

Test plan

• Start the API and open http://localhost:8000/maps
• Verify the dataset dropdown populates from /stac/catalog.json
• Select a published dataset — map zooms to extent, ZarrLayer renders
• Move the time slider — layer updates to the correct time step
• Check that /stac/collections/{id} includes renders and the Render extension URL in stac_extensions
• Verify a dataset template without a display block produces no renders in the collection

Closes #66