UX: adds chat send shortcut user preference (#30473)

Users can now decide if they want to send a message on:
- <kbd>enter</kbd>
- <kbd>meta + enter</kbd>

If you choose <kbd>meta + enter</kbd>, <kbd>enter</kbd> will add a
linebreak.

<img width="192" alt="Screenshot 2025-01-21 at 12 57 48"
src="https://github.com/user-attachments/assets/abfd6f8b-83b3-4e6f-be67-8f63d536ca8a"
/>

plugins/chat/lib/chat/user_option_extension.rb

@@ -39,6 +39,10 @@ module Chat
         @chat_separate_sidebar_mode ||= { default: 0, never: 1, always: 2, fullscreen: 3 }
       end
 
+      def base.chat_send_shortcut
+        @chat_send_shortcut ||= { enter: 0, meta_enter: 1 }
+      end
+
       # Avoid attempting to override when autoloading
       if !base.method_defined?(:chat_separate_sidebar_mode_default?)
         base.enum :chat_separate_sidebar_mode,
@@ -46,6 +50,10 @@ module Chat
                   prefix: "chat_separate_sidebar_mode"
       end
 
+      if !base.method_defined?(:chat_send_shortcut_default?)
+        base.enum :chat_send_shortcut, base.chat_send_shortcut, prefix: "chat_send_shortcut"
+      end
+
       if !base.method_defined?(:show_thread_title_prompts?)
         base.attribute :show_thread_title_prompts, :boolean, default: true
       end