/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/bots/service.go

@@ -82,6 +82,7 @@ type verificationApplications interface {
 // because a service bot must not reach into the file layer for anything else.
 type botAvatarStore interface {
 	SetAvatarFromExistingPhoto(ctx context.Context, ownerType domain.PeerType, ownerID, sourcePhotoID int64, date int) (domain.Photo, error)
+	PeerHasAvatar(ctx context.Context, ownerType domain.PeerType, ownerID int64) (bool, error)
 }
 
 // The third-party verification ports live in verifierbot.go