feat: admin image generation via Cloudflare Workers AI

- Add cfai/image.go: ImageGenClient with GenerateImage, GenerateImageFromReference, AllImageModels (9 models)
- Add handlers_image.go: GET /api/admin/image-gen/models + POST /api/admin/image-gen (JSON + multipart)
- Wire ImageGen client in main.go + server.go Dependencies
- Add admin/image-gen SvelteKit page: type toggle, model selector, prompt, reference img2img, advanced options, result panel, history, save-as-cover, download
- Add SvelteKit proxy route api/admin/image-gen forwarding to Go backend
- Add admin_nav_image_gen message key to all 5 locale files
- Add Image Gen nav link to admin layout

ui/messages/ru.json

@@ -362,6 +362,7 @@
   "admin_nav_audio": "Аудио",
   "admin_nav_translation": "Перевод",
   "admin_nav_changelog": "Изменения",
+  "admin_nav_image_gen": "Image Gen",
   "admin_nav_feedback": "Отзывы",
   "admin_nav_errors": "Ошибки",
   "admin_nav_analytics": "Аналитика",