feat: add image search with Bing, DuckDuckGo, and Qwant engines

Three new image search engines:
- bing_images: Bing Images via RSS endpoint
- ddg_images: DuckDuckGo Images via VQD API
- qwant_images: Qwant Images via v3 search API

Frontend:
- Image grid layout with responsive columns
- image_item template with thumbnail, title, and source metadata
- Hover animations and lazy loading
- Grid activates automatically when category=images

Backend:
- category=images routes to image engines via planner
- Image engines registered in factory and engine allowlist
- extractImgSrc helper for parsing thumbnail URLs from HTML
- IsImageSearch flag on PageData for template layout switching

internal/engines/factory.go

@@ -73,5 +73,9 @@ func NewDefaultPortedEngines(client *http.Client, cfg *config.Config) map[string
 			apiKey:  youtubeAPIKey,
 			baseURL: "https://www.googleapis.com",
 		},
+		// Image engines
+		"bing_images":  &BingImagesEngine{client: client},
+		"ddg_images":   &DuckDuckGoImagesEngine{client: client},
+		"qwant_images": &QwantImagesEngine{client: client},
 	}
 }