chore(platform): assets app + config Android/iOS + screenshots

- Icônes app (hdpi/mdpi/xhdpi/xxhdpi/xxxhdpi + iOS AppIcon) régénérées
- Launch screens Android + iOS actualisés
- ios/Runner : Info.plist, LaunchScreen.storyboard, project.pbxproj
- android/gradle.properties, drawables, styles (values + values-night)
- assets/images : branding/logo
- Screenshots Flutter (flutter_01-04.png) + script PowerShell start-emulators

android/app/src/main/res/drawable-night/launch_background.xml

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="utf-8"?>
+<layer-list xmlns:android="http://schemas.android.com/apk/res/android">
+    <item>
+        <bitmap android:gravity="fill" android:src="@drawable/background"/>
+    </item>
+    <item>
+        <bitmap android:gravity="center" android:src="@drawable/splash"/>
+    </item>
+</layer-list>