Add the Data Library: live regional datasets, tiles and packs

DATA LAYERS keeps its curated core layers; regional and niche sources
live in the Data Library, opened from Browse data library at the foot
of the panel. Each entry is a JSON manifest in src/library/entries/
pointing at a live upstream (query API, provider-hosted file or tile
endpoint), so nothing is bundled and a self-hosted instance hosts no
data. Sources without CORS go through one allowlisted /api/library
route. Kinds: dataset (GeoJSON, CSV, JSON records, ArcGIS, partitioned
GeoParquet), tiles (XYZ, WMS/ArcGIS bbox export, terrarium relief) and
pack. Selections persist and travel in share links as lib=, separate
from the core layer tokens. Library rows get Zoom and Remove icon
chips that work while the layer is off, rendered through RailChips.
Strings are translated in every locale.

.env.example

@@ -280,6 +280,11 @@ AISSTREAM_API_KEY=
 # CCTV_PREFER_AUSTIN=1                                 # prioritize Austin cameras in the registry
 # CCTV_FORCE_AUSTIN=                                   # force live default sources when a source-pack JSON is configured
 
+# Optional: Data Library (DATA LAYERS ▸ Browse data library). Every entry is
+# fetched live from its provider; nothing is hosted here. See docs/DATA-LIBRARY.md.
+# GEV_LIBRARY_ENABLE=                                  # comma-separated ids of operator opt-in entries to serve (packs use their host's own switch, e.g. CCTV_NYC_ENABLED)
+# GEV_LIBRARY_DISABLE=                                 # comma-separated ids to remove from this instance entirely
+
 # Optional: comma-separated Overpass URLs; use only instances you run or pay for.
 # Also enables voice place search (osm_query, "hospitals in Kathmandu").
 OVERPASS_UPSTREAMS=