feat(telegram,game): single bot + per-user variant preferences

Collapse the two per-language Telegram bots into one unified bot and
replace language-based variant gating with explicit per-user variant
preferences.

- Telegram: one bot; drop service_language and the supported_languages
  set everywhere (DB, account, auth, FlatBuffers Session wire, gateway,
  connector proto). The single bot renders chat and out-of-app push in
  the recipient's preferred_language; remove the game-language push
  routing override (notify Intent.Language / push Event.language).
- Preferences: new accounts.variant_preferences (text[], DB default
  {erudit_ru}, CHECK non-empty + subset of the three variants). Gates
  the New Game picker, vs-AI and the friend invitation the player
  creates, enforced server-side (HTTP 400 otherwise); an invited friend
  may still accept any variant. Edited on the Settings screen; variants
  are Erudit-first everywhere.
- Admin: drop the per-bot language selectors (broadcast / send-to-user)
  and the feedback channel_lang column/field.
- Env/CI: collapse TELEGRAM_BOT_TOKEN_{EN,RU}, TELEGRAM_GAME_CHANNEL_ID_{EN,RU},
  VITE_TELEGRAM_LINK{,_EN,_RU} and VITE_TELEGRAM_GAME_CHANNEL_NAME_{EN,RU}
  to single unsuffixed names; drop GATEWAY_DEFAULT_SUPPORTED_LANGUAGES.
- Docs updated (ARCHITECTURE, FUNCTIONAL + _ru, platform/telegram, gateway,
  backend, ui, UI_DESIGN, PRERELEASE).

The migration squash is deferred to a follow-up PR.

backend/internal/account/profile_test.go

@@ -3,6 +3,7 @@ package account
 import (
 	"context"
 	"errors"
+	"slices"
 	"strings"
 	"testing"
 	"time"
@@ -16,7 +17,7 @@ import (
 // offset/IANA timezone), not just their unit tests in validate_test.go.
 func TestUpdateProfileValidation(t *testing.T) {
 	s := &Store{}
-	base := ProfileUpdate{DisplayName: "Kaya", PreferredLanguage: "en", TimeZone: "UTC"}
+	base := ProfileUpdate{DisplayName: "Kaya", PreferredLanguage: "en", TimeZone: "UTC", VariantPreferences: []string{"erudit_ru"}}
 	hm := func(h, m int) time.Time { return time.Date(0, 1, 1, h, m, 0, 0, time.UTC) }
 	tests := []struct {
 		name string
@@ -28,6 +29,8 @@ func TestUpdateProfileValidation(t *testing.T) {
 		{"over-long name", func(p *ProfileUpdate) { p.DisplayName = strings.Repeat("x", maxDisplayName+1) }},
 		{"bad name layout", func(p *ProfileUpdate) { p.DisplayName = "Bad__Name" }},
 		{"away over 12h", func(p *ProfileUpdate) { p.AwayStart, p.AwayEnd = hm(8, 0), hm(21, 0) }},
+		{"empty variant preferences", func(p *ProfileUpdate) { p.VariantPreferences = nil }},
+		{"unknown variant preference", func(p *ProfileUpdate) { p.VariantPreferences = []string{"chess"} }},
 	}
 	for _, tc := range tests {
 		t.Run(tc.name, func(t *testing.T) {
@@ -39,3 +42,22 @@ func TestUpdateProfileValidation(t *testing.T) {
 		})
 	}
 }
+
+// TestValidateVariantPreferences checks the cleaning of a profile's variant set:
+// duplicates collapse, the result is canonically ordered (Erudit, Russian Scrabble,
+// English) regardless of input order, and an empty or unknown set is rejected.
+func TestValidateVariantPreferences(t *testing.T) {
+	got, err := validateVariantPreferences([]string{"scrabble_en", "erudit_ru", "scrabble_en"})
+	if err != nil {
+		t.Fatalf("validate: %v", err)
+	}
+	if want := []string{"erudit_ru", "scrabble_en"}; !slices.Equal(got, want) {
+		t.Fatalf("got %v, want %v", got, want)
+	}
+	if _, err := validateVariantPreferences(nil); !errors.Is(err, ErrInvalidProfile) {
+		t.Fatalf("empty err = %v, want ErrInvalidProfile", err)
+	}
+	if _, err := validateVariantPreferences([]string{"chess"}); !errors.Is(err, ErrInvalidProfile) {
+		t.Fatalf("unknown err = %v, want ErrInvalidProfile", err)
+	}
+}