chore: migrate mail themes to shared framework and locked upstream inputs

tools/cli/preview.go

@@ -66,9 +66,9 @@ func runPreview(c *cli.Context) error {
 		}
 	}
 
-	result := preview.RenderAll(folder, cfg, themeFilter)
-	if result == nil {
-		return fmt.Errorf("no results produced")
+	result, err := preview.RenderAll(folder, cfg, themeFilter)
+	if err != nil {
+		return err
 	}
 
 	if err := preview.WriteRenderedJS(result, outputPath); err != nil {
@@ -78,7 +78,7 @@ func runPreview(c *cli.Context) error {
 	preview.PrintDetailedSummary(result, folder, cfg)
 
 	fmt.Printf("\033[32m[I]\033[0m [Preview] Wrote %d themes x %d templates to %s\n",
-		len(result.Summaries), len(cfg.Templates), outputPath)
+		len(result.Themes), len(cfg.Templates), outputPath)
 
 	return nil
 }