feat(train): OpenFreeMap's dark style in a dark theme, and MAP_DARK_STYLE_URL for a provider's

The maps follow the theme where the source names a dark style, which
only the instance's own map did: OpenFreeMap, the default source, had
none, so a dark phone showed Liberty's bright map. The default now
hands OpenFreeMap's Dark style beside Liberty, and an operator setting
MAP_STYLE_URL may set MAP_DARK_STYLE_URL (HTTPS, else none) beside it,
passed on by compose.train.yaml and named in .env.example.
ADR 0030, D65.

Whats-New: The map is dark in a dark theme.
Whats-New-De: Die Karte ist im dunklen Design dunkel.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/train/halkyon_train_server/test/unit/map/map_source_test.dart

@@ -11,6 +11,8 @@ void main() {
     expect(source.styleUrl, 'https://tiles.openfreemap.org/styles/liberty');
     expect(source.attribution, contains('OpenStreetMap'));
     expect(source.aboutUrl, 'https://openfreemap.org');
+    // Its dark style in a dark theme (D65).
+    expect(source.darkStyleUrl, 'https://tiles.openfreemap.org/styles/dark');
   });
 
   test("an operator's provider is named as they set it", () {
@@ -31,6 +33,26 @@ void main() {
     expect(source.aboutUrl, 'https://www.maptiler.com/privacy-policy/');
   });
 
+  test("an operator's provider has a dark style where they set one over "
+      'HTTPS (D65), none otherwise', () {
+    const style = 'https://maps.example.org/light.json';
+    expect(
+      mapSourceFrom(const {
+        'MAP_STYLE_URL': style,
+        'MAP_DARK_STYLE_URL': 'https://maps.example.org/dark.json',
+      }).darkStyleUrl,
+      'https://maps.example.org/dark.json',
+    );
+    for (final dark in [null, '', 'http://maps.example.org/dark.json']) {
+      expect(
+        mapSourceFrom({'MAP_STYLE_URL': style, 'MAP_DARK_STYLE_URL': ?dark})
+            .darkStyleUrl,
+        isNull,
+        reason: dark,
+      );
+    }
+  });
+
   test('a provider without a name is named by its host, and credits '
       'OpenStreetMap', () {
     final source = mapSourceFrom(const {