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>

android/app/src/main/res/mipmap-anydpi-v26/ic_launcher.xml

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="utf-8"?>
+<adaptive-icon xmlns:android="http://schemas.android.com/apk/res/android">
+  <background android:drawable="@color/ic_launcher_background"/>
+  <foreground>
+      <inset
+          android:drawable="@drawable/ic_launcher_foreground"
+          android:inset="16%" />
+  </foreground>
+</adaptive-icon>

android/app/src/main/res/values/colors.xml

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="utf-8"?>
+<resources>
+    <color name="ic_launcher_background">#080A12</color>
+</resources>