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/results_inner.html

@@ -19,13 +19,25 @@
 
 <div id="urls" role="main">
   {{if .Results}}
+  {{if .IsImageSearch}}
+  <div class="image-grid">
+    {{range .Results}}
+    {{if eq .Template "images"}}
+    {{template "image_item" .}}
+    {{end}}
+    {{end}}
+  </div>
+  {{else}}
   {{range .Results}}
   {{if eq .Template "videos"}}
   {{template "video_item" .}}
+  {{else if eq .Template "images"}}
+  {{template "image_item" .}}
   {{else}}
   {{template "result_item" .}}
   {{end}}
   {{end}}
+  {{end}}
   {{else if not .Answers}}
   <div class="no-results">
     <div class="no-results-icon">🔍</div>