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/search/request_params.go

@@ -35,6 +35,8 @@ var knownEngineNames = map[string]bool{
 	"braveapi": true, "brave": true, "qwant": true,
 	"duckduckgo": true, "github": true, "reddit": true,
 	"bing": true, "google": true, "youtube": true,
+	// Image engines
+	"bing_images": true, "ddg_images": true, "qwant_images": true,
 }
 
 // validateEngines filters engine names against the known registry.