fix(prefs): persist favicon choice and apply to HTML results

- Save favicon cookie on POST /preferences; reflect selection in template
- Add getFaviconService helper; pass favicon service into FromResponse
- Compute ResultView.FaviconIconURL (none/google/duckduckgo/self proxy)
- Update result_item and video_item templates; add httpapi/views tests

Made-with: Cursor

internal/views/templates/result_item.html

@@ -4,8 +4,8 @@
     <a href="{{.URL}}" target="_blank" rel="noopener noreferrer">{{.SafeTitle}}</a>
   </div>
   <div class="result_url">
-    {{if .Domain}}
-    <img class="result-favicon" src="/favicon/{{.Domain}}" alt="" loading="lazy" width="14" height="14">
+    {{if .FaviconIconURL}}
+    <img class="result-favicon" src="{{.FaviconIconURL}}" alt="" loading="lazy" width="14" height="14">
     {{end}}
     <a href="{{.URL}}" target="_blank" rel="noopener noreferrer">{{.URL}}</a>
     <span class="engine-badge" data-engine="{{.Engine}}">{{.Engine}}</span>