Migrate flutter_markdown → flutter_markdown_plus; fix app label

flutter_markdown was discontinued; flutter_markdown_plus is the
maintained drop-in replacement (same API, updated imports only).

Also set android:label to "Fabled" so the app appears correctly
on the home screen and in the app drawer instead of "fabled_app".

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/screens/chat/chat_screen.dart

@@ -1,7 +1,7 @@
 import 'dart:math' show min;
 
 import 'package:flutter/material.dart';
-import 'package:flutter_markdown/flutter_markdown.dart';
+import 'package:flutter_markdown_plus/flutter_markdown_plus.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
 import '../../core/exceptions.dart';

lib/screens/notes/note_detail_screen.dart

@@ -1,5 +1,5 @@
 import 'package:flutter/material.dart';
-import 'package:flutter_markdown/flutter_markdown.dart';
+import 'package:flutter_markdown_plus/flutter_markdown_plus.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:go_router/go_router.dart';
 

lib/screens/notes/note_edit_screen.dart

@@ -1,5 +1,5 @@
 import 'package:flutter/material.dart';
-import 'package:flutter_markdown/flutter_markdown.dart';
+import 'package:flutter_markdown_plus/flutter_markdown_plus.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:go_router/go_router.dart';