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.lock

@@ -17,6 +17,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "13.0.0"
+  archive:
+    dependency: transitive
+    description:
+      name: archive
+      sha256: a96e8b390886ee8abb49b7bd3ac8df6f451c621619f52a26e815fdcf568959ff
+      url: "https://pub.dev"
+    source: hosted
+    version: "4.0.9"
   args:
     dependency: transitive
     description:
@@ -117,10 +125,10 @@ packages:
     dependency: transitive
     description:
       name: cli_util
-      sha256: "5909d2c6b66817222779e1eedc19e0e28b76d1df7bd9856a4792ccb9881df358"
+      sha256: ff6785f7e9e3c38ac98b2fb035701789de90154024a75b6cb926445e83197d1c
       url: "https://pub.dev"
     source: hosted
-    version: "0.5.1"
+    version: "0.4.2"
   clock:
     dependency: transitive
     description:
@@ -278,6 +286,14 @@ packages:
     description: flutter
     source: sdk
     version: "0.0.0"
+  flutter_launcher_icons:
+    dependency: "direct dev"
+    description:
+      name: flutter_launcher_icons
+      sha256: "10f13781741a2e3972126fae08393d3c4e01fa4cd7473326b94b72cf594195e7"
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.14.4"
   flutter_lints:
     dependency: "direct dev"
     description:
@@ -352,6 +368,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "4.1.2"
+  image:
+    dependency: transitive
+    description:
+      name: image
+      sha256: "6300175e00616bbc832e2fc91bfa4d776af5402c81c7151bee6905bb08473c52"
+      url: "https://pub.dev"
+    source: hosted
+    version: "4.9.1"
   image_picker:
     dependency: "direct main"
     description:
@@ -600,6 +624,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "2.3.0"
+  petitparser:
+    dependency: transitive
+    description:
+      name: petitparser
+      sha256: "91bd59303e9f769f108f8df05e371341b15d59e995e6806aefab827b58336675"
+      url: "https://pub.dev"
+    source: hosted
+    version: "7.0.2"
   platform:
     dependency: transitive
     description:
@@ -624,6 +656,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "1.5.2"
+  posix:
+    dependency: transitive
+    description:
+      name: posix
+      sha256: "185ef7606574f789b40f289c233efa52e96dead518aed988e040a10737febb07"
+      url: "https://pub.dev"
+    source: hosted
+    version: "6.5.0"
   pub_semver:
     dependency: transitive
     description:
@@ -837,6 +877,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "1.1.0"
+  xml:
+    dependency: transitive
+    description:
+      name: xml
+      sha256: "67f0aff7be013d107995e9b75bf4e7f2c3ef2dfdb2c8e68024bba0a7fd5756a4"
+      url: "https://pub.dev"
+    source: hosted
+    version: "7.0.1"
   yaml:
     dependency: transitive
     description: