UI-09: switch to CARTO dark map tiles with attribution and cache versioning

Replaces OSM's tan/cream default raster style with CARTO's "Dark Matter"
basemap (both RideMap and the route planner's independent TileLayer), so
the map itself looks like the dark mockups rather than a light basemap
under a dark overlay. Shares one urlTemplate/subdomains/maxNativeZoom
across both call sites instead of two independently-drifting copies.

Re-points the V3-11 tile cache at a versioned directory (tiles/carto_dark_v1)
since TileKey(z, x, y) carries no provider identity and would otherwise
silently serve stale tan tiles cached under the old scheme. Added a test
proving cache isolation holds across provider directories.

Adds a shared TileAttribution widget crediting both OpenStreetMap (CARTO's
style is still built from OSM data) and CARTO, composed into every map
rather than duplicated per screen -- OSM-only credit stopped being
sufficient once a second tile host entered the mix.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012Xki7YAcc2TiN2PRZJ2tXr

test/tile_cache_test.dart

@@ -89,4 +89,26 @@ void main() {
     expect(await reopened.get(const TileKey(5, 10, 10)), isNotNull);
     expect(await reopened.sizeBytes(), 64);
   });
+
+  test('a tile cached under one provider directory is not served from another '
+      '(UI-09)', () async {
+    // `TileKey` carries no provider identity -- (z, x, y) alone can't tell an OSM tan
+    // tile from a CARTO dark one at the same coordinates. `tileCacheProvider` (see
+    // app/providers.dart) relies entirely on giving each tile source its own
+    // subdirectory to prevent a switch from silently serving stale, visually-mismatched
+    // tiles. This proves that isolation actually holds at the `FileTileCache` level.
+    final osm = FileTileCache(
+      directory: Directory('${tempDir.path}/osm'),
+      maxBytes: 1024 * 1024,
+    );
+    final cartoDark = FileTileCache(
+      directory: Directory('${tempDir.path}/carto_dark_v1'),
+      maxBytes: 1024 * 1024,
+    );
+
+    await osm.put(const TileKey(5, 10, 10), bytesOfSize(64));
+
+    expect(await cartoDark.get(const TileKey(5, 10, 10)), isNull,
+        reason: 'a tile cached under the old provider must not leak into the new one');
+  });
 }