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/views/templates/image_item.html

@@ -0,0 +1,15 @@
+{{define "image_item"}}
+<a class="image-result" href="{{.URL}}" target="_blank" rel="noopener noreferrer">
+  <div class="image-thumb">
+    {{if .Thumbnail}}
+    <img src="{{.Thumbnail}}" alt="{{.Title}}" loading="lazy" onerror="this.parentElement.classList.add('image-error')">
+    {{else}}
+    <div class="image-placeholder">🖼️</div>
+    {{end}}
+  </div>
+  <div class="image-meta">
+    <span class="image-title">{{.Title}}</span>
+    {{if .Content}}<span class="image-source">{{.Content}}</span>{{end}}
+  </div>
+</a>
+{{end}}