feat(telegram): link the promo body @username to the Mini App deep link

The promo message body now renders "@<bot>" as an HTML text_link to the same ?startapp deep link the button uses (ParseMode HTML), so tapping the mention opens the seeded Mini App instead of the bot profile. Same payload (campaign start-param, else the forwarded /start payload) backs both the button and the mention.

platform/telegram/internal/promobot/promobot_test.go

@@ -13,25 +13,30 @@ import (
 )
 
 func TestPromoTextLocalization(t *testing.T) {
-	en, enBtn := promoText("en", "ScrabbleBot")
-	if !strings.Contains(en, "@ScrabbleBot") || !strings.Contains(en, "profile settings") {
+	const url = "https://t.me/bot/app?startapp=verudit_ru-scrabble_en"
+	// The @username is rendered as a clickable deep link (the same target as the button),
+	// not a plain mention, so tapping it opens the seeded Mini App.
+	wantLink := `<a href="` + url + `">@ScrabbleBot</a>`
+
+	en, enBtn := promoText("en", "ScrabbleBot", url)
+	if !strings.Contains(en, wantLink) || !strings.Contains(en, "profile settings") {
 		t.Errorf("en text = %q", en)
 	}
 	if enBtn != "🤩 I want to play!" {
 		t.Errorf("en button = %q", enBtn)
 	}
 
-	ru, ruBtn := promoText("ru-RU", "ScrabbleBot")
-	if !strings.Contains(ru, "@ScrabbleBot") || !strings.Contains(ru, "Откройте") {
+	ru, ruBtn := promoText("ru-RU", "ScrabbleBot", url)
+	if !strings.Contains(ru, wantLink) || !strings.Contains(ru, "Откройте") {
 		t.Errorf("ru text = %q", ru)
 	}
 	if ruBtn != "🤩 Хочу играть!" {
 		t.Errorf("ru button = %q", ruBtn)
 	}
 
-	// An unknown language falls back to English.
-	if got, _ := promoText("de", "B"); !strings.Contains(got, "Open @B") {
-		t.Errorf("fallback text = %q, want English", got)
+	// An unknown language falls back to English, still with the linked mention.
+	if got, _ := promoText("de", "B", url); !strings.Contains(got, "Open ") || !strings.Contains(got, `">@B</a>`) {
+		t.Errorf("fallback text = %q, want English with a linked mention", got)
 	}
 }
 
@@ -93,8 +98,8 @@ func TestHandleStartReplies(t *testing.T) {
 	if api.chatID != "42" {
 		t.Errorf("chat_id = %q, want 42", api.chatID)
 	}
-	if !strings.Contains(api.text, "@ScrabbleBot") {
-		t.Errorf("text = %q, want the @mention", api.text)
+	if !strings.Contains(api.text, `<a href="https://t.me/bot/app?startapp=f99">@ScrabbleBot</a>`) {
+		t.Errorf("text = %q, want the @mention linked to the startapp deep link", api.text)
 	}
 	if strings.Contains(api.replyMarkup, "web_app") {
 		t.Errorf("reply_markup = %q has a web_app button; want a url button", api.replyMarkup)