feat(telegram): localized /start welcome with channel & chat follow links

The main bot answered /start with a single English line ("Tap to open Scrabble.").
Localize it: Russian or English by the sender's reported Telegram language
(Message.from.language_code, which the Bot API carries on the message itself — there is
no separate user-update event — English fallback), with the longer welcome copy and a
localized launch button ("Открыть «Эрудит»" / "Open “Erudite”").

The welcome links the game channel and the discussion chat by their public @username,
resolved once at startup from the configured TELEGRAM_GAME_CHANNEL_ID / TELEGRAM_CHAT_ID
via getChat and cached. A handle that is unset, private, or unreadable degrades to a
generic noun ("the channel" / "our chat") rather than a dangling "@", so the paragraph
always reads cleanly (the bot's info screen still lists the real links). Adds
GameChannelID to bot.Config (wired from the existing config) for the channel handle.

Tests: startText localization + handle embedding + per-slot generic fallback; handleStart
language selection; resolveWelcomeHandles. README updated.

platform/telegram/internal/bot/bot_test.go

@@ -29,6 +29,10 @@ func (f *fakeBotAPI) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		f.text = r.FormValue("text")
 		f.replyMarkup = r.FormValue("reply_markup")
 		io.WriteString(w, `{"ok":true,"result":{"message_id":1}}`)
+	case strings.HasSuffix(r.URL.Path, "/getChat"):
+		// Echo the requested id into the username so a resolver test can tell the
+		// channel lookup from the chat lookup.
+		io.WriteString(w, `{"ok":true,"result":{"id":-100,"type":"channel","username":"u`+r.FormValue("chat_id")+`"}}`)
 	default:
 		io.WriteString(w, `{"ok":true,"result":true}`)
 	}
@@ -105,7 +109,7 @@ func TestTestEnvironmentRoutesGetMe(t *testing.T) {
 }
 
 func TestHandleStartRepliesPrivateOnly(t *testing.T) {
-	t.Run("private replies", func(t *testing.T) {
+	t.Run("private replies in english by default", func(t *testing.T) {
 		api := &fakeBotAPI{}
 		b := newTestBot(t, api)
 		b.handleStart(context.Background(), b.api, &models.Update{Message: &models.Message{
@@ -114,6 +118,35 @@ func TestHandleStartRepliesPrivateOnly(t *testing.T) {
 		if api.chatID != "42" || !strings.Contains(api.replyMarkup, "web_app") {
 			t.Errorf("private /start: chat=%q markup=%q, want a web_app reply", api.chatID, api.replyMarkup)
 		}
+		// No reported language -> English welcome + English button.
+		if !strings.Contains(api.text, "Hi!") {
+			t.Errorf("text = %q, want the English welcome", api.text)
+		}
+		if !strings.Contains(api.replyMarkup, "Open") {
+			t.Errorf("reply_markup = %q, want the English button", api.replyMarkup)
+		}
+	})
+	t.Run("uses the sender's reported language", func(t *testing.T) {
+		api := &fakeBotAPI{}
+		b := newTestBot(t, api)
+		b.handleStart(context.Background(), b.api, &models.Update{Message: &models.Message{
+			Chat: models.Chat{ID: 42, Type: models.ChatTypePrivate}, Text: "/start",
+			From: &models.User{ID: 7, LanguageCode: "ru"},
+		}})
+		if !strings.Contains(api.text, "Привет!") {
+			t.Errorf("text = %q, want the Russian welcome for a ru sender", api.text)
+		}
+	})
+	t.Run("embeds resolved follow handles", func(t *testing.T) {
+		api := &fakeBotAPI{}
+		b := newTestBot(t, api)
+		b.channelUsername, b.chatUsername = "erudit", "erudite_chat"
+		b.handleStart(context.Background(), b.api, &models.Update{Message: &models.Message{
+			Chat: models.Chat{ID: 42, Type: models.ChatTypePrivate}, Text: "/start",
+		}})
+		if !strings.Contains(api.text, "@erudit") || !strings.Contains(api.text, "@erudite_chat") {
+			t.Errorf("text = %q, want the follow handles", api.text)
+		}
 	})
 	t.Run("group ignored", func(t *testing.T) {
 		api := &fakeBotAPI{}
@@ -127,6 +160,26 @@ func TestHandleStartRepliesPrivateOnly(t *testing.T) {
 	})
 }
 
+func TestResolveWelcomeHandles(t *testing.T) {
+	api := &fakeBotAPI{}
+	b := newTestBot(t, api)
+	b.channelID, b.chatID = 111, 222
+	b.resolveWelcomeHandles(context.Background())
+	// The fake echoes the requested id into the username, so each lookup is independent.
+	if b.channelUsername != "u111" {
+		t.Errorf("channelUsername = %q, want u111", b.channelUsername)
+	}
+	if b.chatUsername != "u222" {
+		t.Errorf("chatUsername = %q, want u222", b.chatUsername)
+	}
+	// An unset id resolves to no handle (and makes no getChat call).
+	b.channelID = 0
+	b.resolveWelcomeHandles(context.Background())
+	if b.channelUsername != "" {
+		t.Errorf("channelUsername = %q, want empty for id 0", b.channelUsername)
+	}
+}
+
 func TestStartPayload(t *testing.T) {
 	cases := map[string]string{
 		"/start g123":  "g123",