Add test coverage for cmd/vintner and internal/i18n

Both were at 0% coverage. Focused on what's safely testable without
touching the network or filesystem: flag validation (--architecture/
--host-arch, the same guard added in the stability pass), subcommand
dispatch and aliases, help/usage error paths, and - for i18n - full
language-detection table coverage plus a completeness check that
every catalog key has both an EN and RU entry (an English-only or
Russian-only entry would silently degrade rather than fail loudly,
so this is worth locking in). cmd/vintner: 0% -> 28.8%, i18n: 0% ->
94.1%.

cmd/vintner/paths_test.go

@@ -0,0 +1,22 @@
+package main
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestDefaultToolchainDir(t *testing.T) {
+	home, err := os.UserHomeDir()
+	if err != nil {
+		t.Skipf("no home directory available: %v", err)
+	}
+	got, err := defaultToolchainDir()
+	if err != nil {
+		t.Fatalf("defaultToolchainDir() error: %v", err)
+	}
+	want := filepath.Join(home, ".vintner")
+	if got != want {
+		t.Errorf("defaultToolchainDir() = %q, want %q", got, want)
+	}
+}