fix(flutter_port): Mobile-Bugsweep 2026-06-23 (Re-Sweep) — .single StateError

home_screen._pickFile: result.files.single warf StateError bei leerer files-Liste
(result != null). Fix: isEmpty-Guard + result.files.first. Verhaltensgleich fuer
den 1-Datei-Fall, sicher bei leerer Liste. (Muster wie StreamingGuide A4; der
06-20-Sweep kannte es noch nicht.)

Sonst sauber (Markdown-Editor, keine DB -> rejected-Future N/A; mounted-Guards/
catch/Smart-Punctuation bereits 06-20 gefixt; l10n vollstaendig). Regressionstest:
bugsweep_loop_20260623_test.dart. Scope: nur flutter_port. Statische Verifikation.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: lukisch

flutter_port/lib/screens/home_screen.dart

@@ -69,15 +69,18 @@ class _HomeScreenState extends State<HomeScreen> {
     if (!mounted) {
       return;
     }
-    if (result == null || result.files.single.path == null) {
+    // .first statt .single: leere files-Liste (result != null) wirft sonst StateError.
+    if (result == null ||
+        result.files.isEmpty ||
+        result.files.first.path == null) {
       setState(() {
         _isBusy = false;
       });
       return;
     }
 
     try {
-      final text = await File(result.files.single.path!).readAsString();
+      final text = await File(result.files.first.path!).readAsString();
       if (!mounted) {
         return;
       }
@@ -86,8 +89,8 @@ class _HomeScreenState extends State<HomeScreen> {
       setState(() {
         _hasError = false;
         _hasUnsavedChanges = false;
-        _currentFilePath = result.files.single.path;
-        _currentFileName = result.files.single.name;
+        _currentFilePath = result.files.first.path;
+        _currentFileName = result.files.first.name;
         _isBusy = false;
       });
     } catch (e, stackTrace) {

flutter_port/test/bugsweep_loop_20260623_test.dart

@@ -0,0 +1,18 @@
+// Regressionstest fuer den Mobile-Bugsweep 2026-06-23 (CleanMarkdown/flutter_port, RE-SWEEP).
+//
+// Statische Quelltext-Assertion, da `flutter`/`dart` auf der Workstation nicht im
+// PATH ist (kein Laufzeit-Harness). Laut /bugsweep-Skill valide. Red-on-revert.
+import 'dart:io';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  final homeSrc = File('lib/screens/home_screen.dart').readAsStringSync();
+
+  test('_pickFile nutzt nicht mehr .single (StateError-Schutz bei leerer Liste)',
+      () {
+    expect(homeSrc.contains('result.files.single'), isFalse,
+        reason: '.single wirft StateError bei leerer files-Liste (result != null)');
+    expect(homeSrc.contains('result.files.isEmpty'), isTrue,
+        reason: 'isEmpty-Guard + .first ersetzen das unsichere .single');
+  });
+}