feat(theming): apply updates overlay theme_overrides.__active

internal/theming/theming.go

@@ -140,6 +140,52 @@ func Apply(cfg config.Config, themeID string) error {
 			return fmt.Errorf("write %s: %w", tc.DestRel, err)
 		}
 	}
-	_ = cfg // overlay update lands in Task 3
+	_ = cfg // overlay carries config state; overlay file update below
+	if err := updateOverlayActive(th.ID); err != nil {
+		return fmt.Errorf("update overlay: %w", err)
+	}
 	return nil
 }
+
+// updateOverlayActive rewrites ~/.config/archduino/overlay.yaml so that
+// theme_overrides.__active reflects the selected theme, preserving all
+// other keys via a generic map round-trip.
+func updateOverlayActive(themeID string) error {
+	home, err := os.UserHomeDir()
+	if err != nil {
+		return err
+	}
+	path := filepath.Join(home, ".config", "archduino", "overlay.yaml")
+
+	var root map[string]any
+	if data, err := os.ReadFile(path); err == nil {
+		if err := yaml.Unmarshal(data, &root); err != nil {
+			return fmt.Errorf("parse overlay: %w", err)
+		}
+	}
+	if root == nil {
+		root = map[string]any{}
+	}
+
+	if _, ok := root["version"]; !ok {
+		root["version"] = 1
+	}
+
+	var overrides map[string]any
+	if raw, ok := root["theme_overrides"]; ok {
+		if m, ok := raw.(map[string]any); ok {
+			overrides = m
+		}
+	}
+	if overrides == nil {
+		overrides = map[string]any{}
+	}
+	overrides["__active"] = themeID
+	root["theme_overrides"] = overrides
+
+	out, err := yaml.Marshal(root)
+	if err != nil {
+		return err
+	}
+	return platform.WriteUserFile(path, out, 0o644)
+}