feat(ui): make favicons user-configurable, off by default

- Add favicon service preference: None (default), Google, DuckDuckGo
- result_item.html: remove hardcoded Google favicon src, defer to JS
- applyFavicon() reads data-domain attr and sets src or display:none
- Privacy-by-default: users must explicitly opt in to any favicon service
- Add favicon selector to both the settings panel and preferences page

internal/views/templates/result_item.html

@@ -4,7 +4,7 @@
     <a href="{{.URL}}" target="_blank" rel="noopener noreferrer">{{.SafeTitle}}</a>
   </div>
   <div class="result_url">
-    <img class="result-favicon" src="https://www.google.com/s2/favicons?domain={{.URL | urlquery}}&sz=32" alt="" loading="lazy" onerror="this.style.display='none'">
+    <img class="result-favicon" data-domain="{{.URL}}" src="" alt="" loading="lazy" style="display:none">
     <a href="{{.URL}}" target="_blank" rel="noopener noreferrer">{{.URL}}</a>
     <span class="engine-badge" data-engine="{{.Engine}}">{{.Engine}}</span>
   </div>