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>

deploy/compose.train.yaml

@@ -45,6 +45,7 @@ services:
       OPEN_METEO_API_KEY: ${OPEN_METEO_API_KEY:-}
       # The map source (D22); empty: OpenFreeMap.
       MAP_STYLE_URL: ${MAP_STYLE_URL:-}
+      MAP_DARK_STYLE_URL: ${MAP_DARK_STYLE_URL:-}
       MAP_SOURCE_NAME: ${MAP_SOURCE_NAME:-}
       MAP_ATTRIBUTION: ${MAP_ATTRIBUTION:-}
       MAP_ABOUT_URL: ${MAP_ABOUT_URL:-}