chore(docs): update directory tree to reflect 10 visual styles

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -107,12 +107,8 @@ Then open `preview/index.html` directly in a browser. No server required.
 
 ```
 gitea-mail-templates/
-├── themes/               # 5 visual styles, 11 .tmpl files each
-│   ├── horizon/          # Enterprise / Corporate
-│   ├── terminal/         # Developers / Tech
-│   ├── ember/            # Community / Open Source
-│   ├── bloom/            # Creative / Startup
-│   └── heritage/         # Education / Research
+├── themes/               # 10 visual styles, 11 .tmpl files each
+│   ├── ...               # Custom styles are added here as separate directories
 ├── preview/              # Live preview SPA
 │   ├── index.html        # Style/template/client/viewport switcher
 │   └── rendered.js       # Pre-rendered templates (generated by tools/)