refactored broad range of the application, split files, modularized calendar and file views, centralized bottom sheets and clipboard handling, and implemented unit test coverage

lib/utils/clipboard_helper.dart

@@ -0,0 +1,19 @@
+import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
+
+/// Copies [text] to the system clipboard and shows a SnackBar confirmation.
+/// Safe to await: respects context lifecycle via the provided [context].
+Future<void> copyToClipboard(
+  BuildContext context,
+  String text, {
+  String successMessage = 'In Zwischenablage kopiert',
+}) async {
+  await Clipboard.setData(ClipboardData(text: text));
+  if (!context.mounted) return;
+  ScaffoldMessenger.of(context).showSnackBar(
+    SnackBar(
+      content: Text(successMessage),
+      duration: const Duration(seconds: 2),
+    ),
+  );
+}