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

ios/Runner/Assets.xcassets/LaunchBackground.imageset/Contents.json

@@ -0,0 +1,22 @@
+{
+  "images" : [
+    {
+      "filename" : "background.png",
+      "idiom" : "universal"
+    },
+    {
+      "appearances" : [
+        {
+          "appearance" : "luminosity",
+          "value" : "dark"
+        }
+      ],
+      "filename" : "darkbackground.png",
+      "idiom" : "universal"
+    }
+  ],
+  "info" : {
+    "author" : "xcode",
+    "version" : 1
+  }
+}