Force French locale on master and rebrand header tag

- keycloak-init now enables i18n on master with French as the only
  supported locale and the default, so all login pages render in fr.
- Replace dynamic realm.displayName tag (which showed 'Keycloak') with
  hardcoded '// Connexion sécurisée' in the theme header.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -83,7 +83,10 @@ services:
           -s "smtpServer.ssl=false" \
           -s "smtpServer.user=$$SMTP_USER" \
           -s "smtpServer.password=$$SMTP_PASSWORD" \
-          -s "loginTheme=$$LOGIN_THEME"
+          -s "loginTheme=$$LOGIN_THEME" \
+          -s "internationalizationEnabled=true" \
+          -s 'supportedLocales=["fr"]' \
+          -s "defaultLocale=fr"
         echo "Master realm configured."
     networks:
       - keycloak-net