feat: écran détail serveur (stats CPU/RAM + image importable RustFS)

- carte cliquable → ServerDetailScreen : bannière (image ou thème), statut,
  connexion/ouvrir, joueurs/MOTD, jauges CPU/RAM/réseau (docker stats),
  import/suppression d'image (galerie → backend → RustFS).
- api_service: stats(), server(), imageBytes(), uploadImage(), deleteImage().
- deps: image_picker.

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

lib/models.dart

@@ -16,6 +16,7 @@ class GameServer {
   final int? maxPlayers;
   final String? motd;
   final bool hasIcon;
+  final bool hasImage;
 
   const GameServer({
     required this.id,
@@ -32,6 +33,7 @@ class GameServer {
     this.maxPlayers,
     this.motd,
     this.hasIcon = false,
+    this.hasImage = false,
   });
 
   factory GameServer.fromJson(Map<String, dynamic> json) => GameServer(
@@ -49,6 +51,7 @@ class GameServer {
         maxPlayers: json['max_players'] as int?,
         motd: json['motd'] as String?,
         hasIcon: json['has_icon'] as bool? ?? false,
+        hasImage: json['has_image'] as bool? ?? false,
       );
 
   bool get isUp => status == 'online' || status == 'running';