feat(chat): add global pinch-to-zoom text scaling via ChatTextScaleService

lib/services/chat_text_scale_service.dart

@@ -0,0 +1,45 @@
+import 'package:flutter/foundation.dart';
+
+import '../storage/prefs_manager.dart';
+
+/// Client-side accessibility/UI service that exposes a persistent shared text scale
+/// factor. No MeshCoreConnector/RoomServer or protocol interaction occurs, and the
+/// value is saved locally via SharedPreferences so it can be reused in Markdown
+/// viewers, log panels, or other text-heavy widgets without redundant network
+/// dependencies.
+///
+/// Widgets should scope rebuilds using the snippet below so only the scaled text
+/// is rebuilt instead of the entire chat list:
+/// ```dart
+/// context.select<ChatTextScaleService, double>(
+///   (service) => service.scale,
+/// )
+/// ```
+class ChatTextScaleService extends ChangeNotifier {
+  static const _prefKey = 'chat_text_scale';
+  static const double _minScale = 0.8;
+  static const double _maxScale = 1.8;
+
+  double _scale = 1.0;
+
+  double get scale => _scale;
+
+  Future<void> initialize() async {
+    final stored = PrefsManager.instance.getDouble(_prefKey);
+    if (stored != null) {
+      _scale = _clamp(stored);
+    }
+  }
+
+  void setScale(double value) {
+    final next = _clamp(value);
+    if (next == _scale) return;
+    _scale = next;
+    PrefsManager.instance.setDouble(_prefKey, _scale);
+    notifyListeners();
+  }
+
+  void reset() => setScale(1.0);
+
+  double _clamp(double value) => value.clamp(_minScale, _maxScale).toDouble();
+}