/mybots: Edit Bot summary screen, return-to-menu, clickable @mentions

- Edit Bot now shows the current value of every field (Name/About/
  Description/Botpic/Commands) like BotFather, with real botpic status
  via a new PeerHasAvatar port method.
- After editing a field the dialog lands back on a fresh Edit Bot menu
  (working "Back to bot" / "Bots list" buttons) instead of ending, so a
  follow-up button press no longer reports the button as expired.
- Service-bot messages now render @username as a tappable mention entity.

internal/app/files/bot_avatar.go

@@ -50,6 +50,12 @@ func (s *Service) SetAvatarFromExistingPhoto(ctx context.Context, ownerType doma
 	return photo, nil
 }
 
+// PeerHasAvatar reports whether ownerType/ownerID has a current profile photo.
+func (s *Service) PeerHasAvatar(ctx context.Context, ownerType domain.PeerType, ownerID int64) (bool, error) {
+	_, ok, err := s.CurrentProfilePhotoKind(ctx, ownerType, ownerID, domain.ProfilePhotoKindProfile)
+	return ok, err
+}
+
 // photoSourceBytes reads the original image bytes behind a stored photo. Every
 // static size of a photo written by putPhotoStaticSizes points at the same
 // stored object, so any one size yields the full image.