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

lib/widgets/chat_zoom_wrapper.dart

@@ -0,0 +1,45 @@
+import 'package:flutter/material.dart';
+import 'package:provider/provider.dart';
+
+import '../services/chat_text_scale_service.dart';
+
+/// Gesture wrapper that exposes two-finger pinch-to-zoom for chat scrollables.
+/// Double-tap resets the scale. Only the wrapper itself listens to gestures;
+/// child scrollables keep their normal touch handling.
+class ChatZoomWrapper extends StatelessWidget {
+  ChatZoomWrapper({super.key, required this.child, this.onDoubleTap});
+
+  final Widget child;
+  final VoidCallback? onDoubleTap;
+  final _ZoomGestureState _state = _ZoomGestureState();
+
+  @override
+  Widget build(BuildContext context) {
+    final service = context.read<ChatTextScaleService>();
+
+    return GestureDetector(
+      behavior: HitTestBehavior.translucent,
+      onDoubleTap: () {
+        service.reset();
+        onDoubleTap?.call();
+      },
+      onScaleStart: (details) {
+        if (details.pointerCount != 2) return;
+        _state.startScale = service.scale;
+      },
+      onScaleUpdate: (details) {
+        if (details.pointerCount != 2) return;
+        final baseScale = _state.startScale ?? service.scale;
+        service.setScale(baseScale * details.scale);
+      },
+      onScaleEnd: (_) {
+        _state.startScale = null;
+      },
+      child: child,
+    );
+  }
+}
+
+class _ZoomGestureState {
+  double? startScale;
+}