feat(variants): default a registered account to Erudit + Russian Scrabble

New Game opened on a single variant for everyone, which reads poorly on VK
where Russian Scrabble is the familiar game. A registered account now starts
with both Russian-alphabet games, so the picker offers a real choice with no
pre-selection.

A guest stays on Erudit alone and is invited to register for the rest: the
device-local guest has no profile at all, so the client-side fallback has to
keep matching the server. Registering promotes the set, but only while the
guest still carries the untouched guest default. Existing accounts are not
backfilled — the set they carry may be a deliberate choice.

The Telegram promo start-param becomes additive rather than a replacement,
with English Scrabble withheld from a Russian-speaking arrival; otherwise the
English campaign link would have taken Russian Scrabble away from every
account it onboarded.

backend/internal/account/variant_seed_test.go

@@ -35,3 +35,37 @@ func TestSeedVariantsFromStartParam(t *testing.T) {
 		})
 	}
 }
+
+// TestMergeVariantSeed covers resolving what a promo-onboarded account should end up with:
+// the seed only ever widens the current set, English Scrabble is withheld from a
+// Russian-speaking arrival, and a merge that changes nothing reports nil so the caller
+// skips the write.
+func TestMergeVariantSeed(t *testing.T) {
+	promo := []string{"erudit_ru", "scrabble_en"} // the campaign link "verudit_ru-scrabble_en"
+	tests := []struct {
+		name     string
+		current  []string
+		seed     []string
+		language string
+		want     []string
+	}{
+		{"russian speaker keeps the default", DefaultVariantPreferences, promo, "ru", nil},
+		{"russian locale variant is still russian", DefaultVariantPreferences, promo, "ru-RU", nil},
+		{"english speaker gains english", DefaultVariantPreferences, promo, "en", []string{"erudit_ru", "scrabble_ru", "scrabble_en"}},
+		{"unsupported language is not russian", DefaultVariantPreferences, promo, "de", []string{"erudit_ru", "scrabble_ru", "scrabble_en"}},
+		{"absent language is not russian", DefaultVariantPreferences, promo, "", []string{"erudit_ru", "scrabble_ru", "scrabble_en"}},
+		{"no seed writes nothing", DefaultVariantPreferences, nil, "en", nil},
+		{"seed already covered writes nothing", DefaultVariantPreferences, []string{"scrabble_ru"}, "ru", nil},
+		{"the russian filter spares the account's own english", []string{"erudit_ru", "scrabble_en"}, promo, "ru", nil},
+		{"invalid seed writes nothing", DefaultVariantPreferences, []string{"chess"}, "en", nil},
+		{"a guest set is widened by the seed", GuestVariantPreferences, []string{"scrabble_ru"}, "ru", DefaultVariantPreferences},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			got := MergeVariantSeed(tc.current, tc.seed, tc.language)
+			if !slices.Equal(got, tc.want) {
+				t.Errorf("MergeVariantSeed(%v, %v, %q) = %v, want %v", tc.current, tc.seed, tc.language, got, tc.want)
+			}
+		})
+	}
+}