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/search/request_params.go

@@ -18,11 +18,13 @@ func ParseSearchRequest(r *http.Request) (SearchRequest, error) {
 
 	format := strings.ToLower(r.FormValue("format"))
 	switch OutputFormat(format) {
-	case FormatJSON, FormatCSV, FormatRSS:
+	case FormatJSON, FormatCSV, FormatRSS, FormatHTML:
+		// explicit format — use as-is
 	default:
-		// MVP: treat everything else as json, except `html` which we accept for compatibility.
-		if format == string(FormatHTML) {
-			// accepted, but not implemented by the server yet
+		// No format specified: default to HTML for browser requests, JSON for API clients.
+		accept := r.Header.Get("Accept")
+		if strings.Contains(accept, "text/html") {
+			format = string(FormatHTML)
 		} else {
 			format = string(FormatJSON)
 		}