feat(assets): icône d'application Android et iOS (ticket #19)

Génère les icônes via flutter_launcher_icons à partir des visuels
icon_full.png / icon_foreground.png : mipmap Android classiques,
icône adaptive (drawable-*/ic_launcher_foreground.png,
mipmap-anydpi-v26/ic_launcher.xml, values/colors.xml) et l'ensemble
AppIcon iOS. Configuration flutter_launcher_icons ajoutée à
pubspec.yaml. flutter analyze propre, 27/27 tests verts, build APK
debug validé, icônes vérifiées visuellement.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

pubspec.yaml

@@ -24,6 +24,7 @@ dev_dependencies:
   flutter_lints: ^6.0.0
   build_runner: ^2.7.0
   drift_dev: ^2.34.2
+  flutter_launcher_icons: ^0.14.4
 
 flutter:
   uses-material-design: true
@@ -34,3 +35,11 @@ flutter:
     - family: Archivo
       fonts:
         - asset: assets/fonts/Archivo-Variable.ttf
+
+flutter_launcher_icons:
+  android: true
+  ios: true
+  image_path: assets/icon/icon_full.png
+  adaptive_icon_background: "#080A12"
+  adaptive_icon_foreground: assets/icon/icon_foreground.png
+  min_sdk_android: 21