Add ESC keyboard shortcut to close email detail view

Adds the ability to close the email detail/thread detail view by pressing
the ESC key, following the same pattern used for closing the composer.

Changes:
- Add closeThreadDetail to MailViewActionShortcutType enum
- Map ESC key to closeThreadDetail in AppShortcutManager
- Handle closeThreadDetail action in HandleMailShortcutActionsExtension

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

lib/features/base/shortcut/app_shortcut_manager.dart

@@ -25,6 +25,8 @@ class AppShortcutManager {
       return MailViewActionShortcutType.newMessage;
     } else if (keysPressed.isOnly(LogicalKeyboardKey.keyU)) {
       return MailViewActionShortcutType.markAsUnread;
+    } else if (keysPressed.isOnly(LogicalKeyboardKey.escape)) {
+      return MailViewActionShortcutType.closeThreadDetail;
     } else {
       return null;
     }

lib/features/base/shortcut/mail/mail_view_action_shortcut_type.dart

@@ -9,7 +9,8 @@ enum MailViewActionShortcutType {
   forward,
   delete,
   newMessage,
-  markAsUnread;
+  markAsUnread,
+  closeThreadDetail;
 
   EmailActionType? getEmailActionType({
     required PresentationEmail currentEmail,
@@ -34,6 +35,8 @@ enum MailViewActionShortcutType {
         return EmailActionType.compose;
       case MailViewActionShortcutType.markAsUnread:
         return EmailActionType.markAsUnread;
+      case MailViewActionShortcutType.closeThreadDetail:
+        return null;
     }
   }
 }