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/html_helpers.go

@@ -72,3 +72,14 @@ func htmlUnescape(s string) string {
 	s = strings.ReplaceAll(s, " ", " ")
 	return s
 }
+
+// extractImgSrc finds the first <img src="..."> in an HTML string and returns
+// the src attribute value.
+func extractImgSrc(html string) string {
+	idx := strings.Index(html, "<img")
+	if idx == -1 {
+		return ""
+	}
+	remaining := html[idx:]
+	return extractAttr(remaining, "src")
+}