fix: resolve web build failures

- Update SDK constraint to >=3.0.0 <4.0.0 (Flutter stable uses Dart 3.x)
- Remove flutter_email_sender (no web support) — replace with mailto: via url_launcher
- support.dart: unified _sendEmail works on both web and mobile

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

pubspec.yaml

@@ -19,7 +19,7 @@ publish_to: 'none' # Remove this line if you wish to publish to pub.dev
 version: 1.0.11+11
 
 environment:
-  sdk: '>=2.19.3 <3.0.0'
+  sdk: '>=3.0.0 <4.0.0'
 
 # Dependencies specify other packages that your package needs in order to work.
 # To automatically upgrade your package dependencies to the latest versions
@@ -55,7 +55,6 @@ dependencies:
   geocoding: ^2.1.0
   url_launcher: ^6.1.10
   community_material_icon: ^5.9.55
-  flutter_email_sender: ^5.2.0
   webview_flutter: ^4.4.1
   responsive_builder: ^0.7.0
   flutter_local_notifications: ^14.1.1