Add tank catalog with USDZ models and tank selection

Introduce TankDefinition/TankCatalog loaded from Tanks.json, bundle the
Drachengas USDZ tank models, add tank selection and preview views, and
update the AR experience, editor and project views to use the catalog.
Includes tests for catalog loading and model selection.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

GasTankAR/Views/ProjectDetailView.swift

@@ -10,6 +10,7 @@ import SwiftData
 
 struct ProjectDetailView: View {
     @Bindable var project: Project
+    @State private var isARPresented = false
 
     var body: some View {
         TabView {
@@ -37,13 +38,11 @@ struct ProjectDetailView: View {
                 )
             }
 
-            Section("Tankauswahl") {
-                TankEditor(tank: $project.tank)
-            }
+            TankSelectionSection(tank: $project.tank)
         }
         .safeAreaInset(edge: .bottom) {
-            NavigationLink {
-                ARExperienceView(project: project)
+            Button {
+                isARPresented = true
             } label: {
                 Label("AR-Modus starten", systemImage: "arkit")
                     .font(.headline)
@@ -53,6 +52,9 @@ struct ProjectDetailView: View {
             .buttonStyle(.borderedProminent)
             .padding()
         }
+        .fullScreenCover(isPresented: $isARPresented) {
+            ARExperienceView(project: project)
+        }
     }
 }