feat(tui): add NvChadTheme field and ThemeByID helper

internal/tui/state_test.go

@@ -70,3 +70,31 @@ func TestSelectedIDsOnlyReturnsChecked(t *testing.T) {
 		t.Fatalf("got %v", got)
 	}
 }
+
+func TestBuiltinThemesCarryNvChadName(t *testing.T) {
+	for _, th := range BuiltinThemes {
+		if th.NvChadTheme == "" {
+			t.Errorf("theme %q has empty NvChadTheme", th.ID)
+		}
+	}
+}
+
+func TestThemeByIDReturnsMatch(t *testing.T) {
+	got, ok := ThemeByID(BuiltinThemes, "dracula")
+	if !ok {
+		t.Fatal("dracula should exist")
+	}
+	if got.ID != "dracula" {
+		t.Fatalf("got id=%q", got.ID)
+	}
+	if got.NvChadTheme == "" {
+		t.Fatal("NvChadTheme empty")
+	}
+}
+
+func TestThemeByIDFallsBackWhenUnknown(t *testing.T) {
+	_, ok := ThemeByID(BuiltinThemes, "does-not-exist")
+	if ok {
+		t.Fatal("unknown id should report ok=false")
+	}
+}