fix(theming): restore gruvbox as default via Default() helper

internal/theming/theming.go

@@ -97,6 +97,21 @@ func List() []Theme {
 	return out
 }
 
+// Default returns the archduino default theme. Used by the wizard's fresh-
+// install default and by callers that resolve an unknown theme ID.
+// The identity here is stable policy: gruvbox is the project's long-standing
+// default (carried over from the pre-Go installer). Changing this changes
+// the out-of-box experience on fresh systems.
+func Default() Theme {
+	if t, ok := Get("gruvbox"); ok {
+		return t
+	}
+	if len(discovered) > 0 {
+		return discovered[0]
+	}
+	return Theme{ID: "gruvbox", DisplayName: "Gruvbox Dark", NvChadTheme: "gruvbox"}
+}
+
 // Get returns the theme with the given ID, if present.
 func Get(id string) (Theme, bool) {
 	for _, t := range discovered {

internal/theming/theming_test.go

@@ -192,3 +192,9 @@ func TestApplyEachThemeWritesStarship(t *testing.T) {
 		})
 	}
 }
+
+func TestDefaultIsGruvbox(t *testing.T) {
+	if got := Default(); got.ID != "gruvbox" {
+		t.Fatalf("Default().ID = %q, want gruvbox", got.ID)
+	}
+}