feat: HTMX + Go Templates HTML frontend

- Add internal/views/ package with embedded templates and static files
- Go html/template with SearXNG-compatible CSS class names
- Dark mode via prefers-color-scheme, responsive layout, print styles
- HTMX integration:
  - Debounced instant search (500ms) on the search input
  - Form submission targets #results via hx-post
  - Pagination buttons are HTMX-powered (swap results div only)
  - HX-Request header detection for fragment vs full page rendering
- Template structure:
  - base.html: full page layout with HTMX script, favicon, CSS
  - index.html: homepage with centered search box
  - results.html: full results page (wraps base + results_inner)
  - results_inner.html: results fragment (HTMX partial + sidebar + pagination)
  - result_item.html: reusable result article partial
- Smart format detection: browser requests (Accept: text/html) default to HTML,
  API clients default to JSON
- Static files served at /static/ from embedded FS (CSS, favicon SVG)
- Index route at GET /
- Empty query on HTML format redirects to homepage
- Custom CSS (gosearch.css): clean, minimal, privacy-respecting aesthetic
  with light/dark mode, responsive breakpoints, print stylesheet
- Add views package tests

internal/views/templates/result_item.html

@@ -0,0 +1,16 @@
+{{define "result_item"}}
+<article class="result">
+  <h3 class="result_header">
+    <a href="{{.URL}}" target="_blank" rel="noopener noreferrer">{{.Title}}</a>
+  </h3>
+  <div class="result_url">
+    <a href="{{.URL}}" target="_blank" rel="noopener noreferrer">{{.URL}}</a>
+  </div>
+  {{if .Content}}
+  <p class="result_content">{{.Content}}</p>
+  {{end}}
+  {{if .Engine}}
+  <div class="result_engine"><span class="engine">{{.Engine}}</span></div>
+  {{end}}
+</article>
+{{end}}