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/welcome_test.go

@@ -0,0 +1,73 @@
+package bot
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestStartTextLocalizesByLanguage(t *testing.T) {
+	cases := []struct {
+		name       string
+		lang       string
+		wantButton string
+		wantSubstr string // a phrase unique to the chosen language body
+	}{
+		{"russian", "ru", "Открыть «Эрудит»", "Привет!"},
+		{"russian region tag", "ru-RU", "Открыть «Эрудит»", "Первая партия"},
+		{"english", "en", "Open “Erudite”", "Hi!"},
+		{"other language falls back to english", "de", "Open “Erudite”", "Hi!"},
+		{"absent language falls back to english", "", "Open “Erudite”", "no more talking"},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			text, button := startText(tc.lang, "erudit", "erudite_chat")
+			if button != tc.wantButton {
+				t.Errorf("button = %q, want %q", button, tc.wantButton)
+			}
+			if !strings.Contains(text, tc.wantSubstr) {
+				t.Errorf("text %q does not contain %q", text, tc.wantSubstr)
+			}
+		})
+	}
+}
+
+func TestStartTextEmbedsFollowHandles(t *testing.T) {
+	for _, lang := range []string{"ru", "en"} {
+		text, _ := startText(lang, "erudit", "erudite_chat")
+		if !strings.Contains(text, "@erudit") || !strings.Contains(text, "@erudite_chat") {
+			t.Errorf("lang %q: follow paragraph missing the handles: %q", lang, text)
+		}
+	}
+}
+
+func TestStartTextFallsBackToGenericWhenHandleMissing(t *testing.T) {
+	// An unresolved handle degrades to a generic noun rather than a dangling "@" — and
+	// only that slot degrades; a resolved sibling still shows its "@username".
+	t.Run("both missing leaves no @", func(t *testing.T) {
+		for _, lang := range []string{"ru", "en"} {
+			text, _ := startText(lang, "", "")
+			if strings.Contains(text, "@") {
+				t.Errorf("lang %q: text shows a dangling @: %q", lang, text)
+			}
+		}
+		// The generic nouns are present in each language.
+		ru, _ := startText("ru", "", "")
+		if !strings.Contains(ru, "на канал") || !strings.Contains(ru, "в нашем чате") {
+			t.Errorf("russian generic fallback missing: %q", ru)
+		}
+		en, _ := startText("en", "", "")
+		if !strings.Contains(en, "Follow the channel") || !strings.Contains(en, "in our group chat") {
+			t.Errorf("english generic fallback missing: %q", en)
+		}
+	})
+	t.Run("only the missing slot degrades", func(t *testing.T) {
+		// Channel resolved, chat missing: the channel keeps its @handle, the chat is generic.
+		en, _ := startText("en", "erudit", "")
+		if !strings.Contains(en, "@erudit") || strings.Contains(en, "@erudite") {
+			t.Errorf("channel handle not shown / chat handle leaked: %q", en)
+		}
+		if !strings.Contains(en, "in our group chat") {
+			t.Errorf("chat slot did not degrade to a generic noun: %q", en)
+		}
+	})
+}