feat: per-engine accent colors in search results

Each engine now has a distinctive color accent applied to its result
card (left border) and engine badge (colored left strip + text).

16 engines mapped to brand-appropriate colors:
Google blue, Bing teal, DDG orange-red, Brave red, Qwant blue,
Wikipedia dark, GitHub purple, Reddit orange-red, YouTube red,
Stack Overflow amber, arXiv crimson, Crossref navy blue.

Pure CSS via data-engine attribute — no JavaScript.

internal/views/templates/result_item.html

@@ -1,12 +1,12 @@
 {{define "result_item"}}
-<article class="result">
+<article class="result" data-engine="{{.Engine}}">
   <div class="result_header">
     <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'">
     <a href="{{.URL}}" target="_blank" rel="noopener noreferrer">{{.URL}}</a>
-    <span class="engine-badge">{{.Engine}}</span>
+    <span class="engine-badge" data-engine="{{.Engine}}">{{.Engine}}</span>
   </div>
   {{if .Content}}
   <p class="result_content">{{.SafeContent}}</p>