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/lib/src/map/map_source_config.dart

@@ -3,10 +3,12 @@ import 'dart:io';
 import 'package:halkyon_train_server/src/generated/protocol.dart';
 
 /// The map source where the operator sets none (ADR 0030, D11): OpenFreeMap,
-/// which takes no key and makes no contract, its Liberty style.
+/// which takes no key and makes no contract, its Liberty style, and its Dark
+/// one in a dark theme (D65).
 final openFreeMap = MapSource(
   name: 'OpenFreeMap',
   styleUrl: 'https://tiles.openfreemap.org/styles/liberty',
+  darkStyleUrl: 'https://tiles.openfreemap.org/styles/dark',
   attribution: 'OpenFreeMap © OpenMapTiles Data from OpenStreetMap',
   aboutUrl: 'https://openfreemap.org',
 );
@@ -21,7 +23,8 @@ MapConfig mapConfig() => mapConfigFrom(Platform.environment);
 /// The map [environment] sets. `MAP_TILES_URL` (D50), the instance's own
 /// Protomaps tiles as an HTTPS `{z}/{x}/{y}` template, drawn by Train's
 /// style, served beside them at `/map/style/light.json` and `dark.json`;
-/// else `MAP_STYLE_URL`, a provider's MapLibre style over HTTPS; else
+/// else `MAP_STYLE_URL`, a provider's MapLibre style over HTTPS, with
+/// `MAP_DARK_STYLE_URL` its dark one (HTTPS, else none, D65); else
 /// [openFreeMap]. Both with `MAP_SOURCE_NAME` (else the host),
 /// `MAP_ATTRIBUTION` (else OpenStreetMap's credit, and Protomaps' for the
 /// instance's tiles) and `MAP_ABOUT_URL` (HTTPS, else none).
@@ -59,6 +62,7 @@ MapConfig mapConfigFrom(Map<String, String> environment) {
     source: MapSource(
       name: set('MAP_SOURCE_NAME') ?? style.host,
       styleUrl: style.toString(),
+      darkStyleUrl: _https(environment['MAP_DARK_STYLE_URL'])?.toString(),
       attribution: set('MAP_ATTRIBUTION') ?? '© OpenStreetMap contributors',
       aboutUrl: about,
     ),

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 {