refactor: Projektstruktur in app/ und backend/ aufgeteilt

.gitignore

@@ -1,4 +1,3 @@
-/target/
 .idea/
 .vscode/
 .settings
@@ -8,14 +7,27 @@
 *.iml
 .DS_Store
 
-# The following files are generated/updated by vaadin-maven-plugin
-node_modules/
-src/main/frontend/generated/
-vite.generated.ts
+# Backend (Spring Boot / Vaadin)
+/backend/target/
+/backend/node_modules/
+/backend/src/main/frontend/generated/
+/backend/vite.generated.ts
+/backend/logs/
+/backend/.env
 
-# Log files
-logs/
+# Flutter app
+/app/.dart_tool/
+/app/build/
+/app/coverage/
+/app/.flutter-plugins
+/app/.flutter-plugins-dependencies
+/app/android/.gradle/
+/app/android/.kotlin/
+/app/android/local.properties
+/app/ios/Pods/
+/app/ios/.symlinks/
+/app/macos/Pods/
+
+# Root build artifacts
+/target/
 *.log
-
-# Environment variables
-.env