/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/mybots_botpic_test.go

@@ -15,6 +15,11 @@ type fakeBotAvatar struct {
 	sourcePhotoID int64
 	calls         int
 	err           error
+	hasAvatar     bool
+}
+
+func (f *fakeBotAvatar) PeerHasAvatar(_ context.Context, _ domain.PeerType, _ int64) (bool, error) {
+	return f.hasAvatar, nil
 }
 
 func (f *fakeBotAvatar) SetAvatarFromExistingPhoto(_ context.Context, ownerType domain.PeerType, ownerID, sourcePhotoID int64, _ int) (domain.Photo, error) {