fix(account): seed a fresh guest's interface language from the client locale

A guest was provisioned with only its detected time zone; its PreferredLanguage
fell to the 'en' column default. The client already sends its locale in the guest
login (GuestLoginRequest.locale), but the gateway dropped it and ProvisionGuest
took no language — so a Russian user's brand-new native guest got English
language-dependent server content (the ad banner, bot messages) until the client's
later language reconcile. Read the locale in the gateway guest handler, thread it
through GuestAuth -> ProvisionGuest, and seed PreferredLanguage from it (validated;
an unsupported/absent value keeps the 'en' default). Wire field already present —
no schema change.

backend/internal/inttest/account_test.go

@@ -229,21 +229,36 @@ func TestProvisionSeedsTimeZone(t *testing.T) {
 		t.Errorf("TimeZone = %q, want UTC fallback for a malformed offset", bad.TimeZone)
 	}
 
-	// A guest is seeded its detected offset; an empty one keeps the UTC default.
-	guest, err := store.ProvisionGuest(ctx, "-05:30")
+	// A guest is seeded its detected offset and interface language; an empty offset keeps the
+	// UTC default and an empty/unsupported language keeps the 'en' default.
+	guest, err := store.ProvisionGuest(ctx, "-05:30", "ru")
 	if err != nil {
 		t.Fatalf("provision guest: %v", err)
 	}
 	if guest.TimeZone != "-05:30" {
 		t.Errorf("guest TimeZone = %q, want the seeded -05:30", guest.TimeZone)
 	}
-	plainGuest, err := store.ProvisionGuest(ctx, "")
+	if guest.PreferredLanguage != "ru" {
+		t.Errorf("guest PreferredLanguage = %q, want the seeded ru", guest.PreferredLanguage)
+	}
+	plainGuest, err := store.ProvisionGuest(ctx, "", "")
 	if err != nil {
 		t.Fatalf("provision plain guest: %v", err)
 	}
 	if plainGuest.TimeZone != "UTC" {
 		t.Errorf("plain guest TimeZone = %q, want UTC default", plainGuest.TimeZone)
 	}
+	if plainGuest.PreferredLanguage != "en" {
+		t.Errorf("plain guest PreferredLanguage = %q, want en default", plainGuest.PreferredLanguage)
+	}
+	// An unsupported locale falls back to the 'en' default rather than persisting a junk value.
+	frGuest, err := store.ProvisionGuest(ctx, "", "fr-FR")
+	if err != nil {
+		t.Fatalf("provision fr guest: %v", err)
+	}
+	if frGuest.PreferredLanguage != "en" {
+		t.Errorf("unsupported-locale guest PreferredLanguage = %q, want en default", frGuest.PreferredLanguage)
+	}
 }
 
 // TestProvisionTelegramUnknownLanguageDefaults checks an unsupported Telegram