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>

android/.gitignore

@@ -0,0 +1,14 @@
+gradle-wrapper.jar
+/.gradle
+/captures/
+/gradlew
+/gradlew.bat
+/local.properties
+GeneratedPluginRegistrant.java
+.cxx/
+
+# Remember to never publicly share your keystore.
+# See https://flutter.dev/to/reference-keystore
+key.properties
+**/*.keystore
+**/*.jks