Scalar Presence: brand-panel login (light/dark), Poppins, splash and launcher icons, fixed API base via dart-define, keep-signed-in, Android project

- Login reworked to the Brand panel layout with a dark-theme switch; 'Scalar Presence' two-line wordmark
- Android project now tracked (was gitignored); pins shared_preferences_android 2.4.27 (2.5.0 crashed with a black screen)
- API base URL fixed per build (--dart-define=API_BASE_URL), no in-app server setting

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

lib/api_service.dart

@@ -111,7 +111,7 @@ class ApiService {
   ApiService(this.settings);
 
   Uri _uri(String p, [Map<String, String>? q]) =>
-      Uri.parse('${settings.baseUrl}$p').replace(queryParameters: q);
+      Uri.parse('${Settings.baseUrl}$p').replace(queryParameters: q);
 
   Map<String, String> get _h => {
         'Content-Type': 'application/json',