Add OpenSeaMap / OpenSnowMap overlay layers (#10)

Two independently toggleable open-data raster overlays — OpenSeaMap
sea marks (buoys, lighthouses, ports) and OpenSnowMap ski pistes —
drawn as Cesium.ImageryLayers directly on the Cesium globe surface.
Same "small array of descriptors" shape as MAP_STACKS in
mapStackController.js.

Visible on the osm/bing-aerial/bing-labels map stacks and invisible
(not broken — the layer's stats say so in plain text) on the default
photoreal stack, since Google Photorealistic 3D Tiles render with the
globe hidden.

OpenRailwayMap and OpenInfraMap, also requested in #10, are
deliberately NOT included in this pass: OpenRailwayMap's public tile
server enforces a Referer allowlist scoped to its own site (confirmed
by direct request — every other Referer, including none, 403s), and
OpenInfraMap serves MapLibre GL vector tiles, not raster XYZ — that
would need a genuine vector-tile rendering pipeline, not a
Cesium.ImageryLayer drop-in. Worth its own dedicated pass.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

src/data/dataCredits.js

@@ -66,6 +66,18 @@ export const DATA_CREDITS = [
     key: 'usgs',
     html: 'Earthquakes: Data courtesy of the U.S. Geological Survey',
   },
+  {
+    key: 'openseamap',
+    html:
+      'Sea marks overlay: ' +
+      '<a href="https://www.openseamap.org" target="_blank" rel="noopener">© OpenSeaMap contributors</a>',
+  },
+  {
+    key: 'opensnowmap',
+    html:
+      'Ski pistes overlay: Map data © OpenStreetMap contributors, SRTM | Map style © ' +
+      '<a href="https://www.opensnowmap.org" target="_blank" rel="noopener">OpenSnowMap.org</a> (CC BY-SA)',
+  },
   {
     key: 'overpass',
     html: