feat: add autocomplete dropdown UI with keyboard nav

- Inline JS in base.html: debounced fetch from /autocompleter on keyup
- Keyboard nav: arrows to navigate, Enter to select, Esc to close
- Highlight matching prefix in suggestions
- Click to select and submit
- Dropdown positioned absolutely below search input
- Dark mode compatible via existing CSS variables

internal/views/templates/base.html

@@ -20,6 +20,123 @@
   <footer>
     <p>Powered by <a href="https://git.ashisgreat.xyz/penal-colony/kafka">kafka</a> — a privacy-respecting, open metasearch engine</p>
   </footer>
+  <script>
+  (function () {
+    'use strict';
+
+    var input = document.getElementById('q');
+    var dropdown = document.getElementById('autocomplete-dropdown');
+    var form = document.getElementById('search-form');
+    var debounceTimer = null;
+    var suggestions = [];
+    var activeIndex = -1;
+    var fetchController = null;
+
+    // Escape regex special chars for highlight matching
+    function escapeRegex(str) {
+      return str.replace(/[.*+?^${}()|[\]\\]/g, '\\$&');
+    }
+
+    // Highlight matching prefix
+    function highlight(text, query) {
+      if (!query) return text;
+      var re = new RegExp('^(' + escapeRegex(query) + ')', 'i');
+      return text.replace(re, '<mark>$1</mark>');
+    }
+
+    function renderDropdown() {
+      if (suggestions.length === 0) {
+        dropdown.classList.remove('open');
+        return;
+      }
+      var html = '';
+      for (var i = 0; i < suggestions.length; i++) {
+        var escaped = highlight(suggestions[i], input.value);
+        html += '<div class="autocomplete-suggestion" data-index="' + i + '">' + escaped + '</div>';
+      }
+      html += '<div class="autocomplete-footer">Press <kbd>↑</kbd><kbd>↓</kbd> to navigate, Enter to select, Esc to close</div>';
+      dropdown.innerHTML = html;
+      dropdown.classList.add('open');
+      activeIndex = -1;
+    }
+
+    function closeDropdown() {
+      dropdown.classList.remove('open');
+      suggestions = [];
+      activeIndex = -1;
+    }
+
+    function selectSuggestion(index) {
+      if (index < 0 || index >= suggestions.length) return;
+      input.value = suggestions[index];
+      closeDropdown();
+      form.submit();
+    }
+
+    function updateActive(newIndex) {
+      var items = dropdown.querySelectorAll('.autocomplete-suggestion');
+      items.forEach(function (el) { el.classList.remove('active'); });
+      if (newIndex >= 0 && newIndex < items.length) {
+        items[newIndex].classList.add('active');
+        items[newIndex].scrollIntoView({ block: 'nearest' });
+      }
+      activeIndex = newIndex;
+    }
+
+    function fetchSuggestions(query) {
+      if (fetchController) fetchController.abort();
+      fetchController = new AbortController();
+      fetch('/autocompleter?q=' + encodeURIComponent(query), { signal: fetchController.signal })
+        .then(function (r) { return r.json(); })
+        .then(function (data) {
+          suggestions = data || [];
+          renderDropdown();
+        })
+        .catch(function (e) {
+          if (e.name !== 'AbortError') suggestions = [];
+          dropdown.classList.remove('open');
+        });
+    }
+
+    input.addEventListener('input', function () {
+      clearTimeout(debounceTimer);
+      var q = input.value.trim();
+      if (q.length < 2) { closeDropdown(); return; }
+      debounceTimer = setTimeout(function () { fetchSuggestions(q); }, 250);
+    });
+
+    input.addEventListener('keydown', function (e) {
+      if (!dropdown.classList.contains('open')) return;
+      var items = dropdown.querySelectorAll('.autocomplete-suggestion');
+      if (e.key === 'ArrowDown') {
+        e.preventDefault();
+        updateActive(Math.min(activeIndex + 1, items.length - 1));
+      } else if (e.key === 'ArrowUp') {
+        e.preventDefault();
+        updateActive(Math.max(activeIndex - 1, -1));
+      } else if (e.key === 'Enter' && activeIndex >= 0) {
+        e.preventDefault();
+        selectSuggestion(activeIndex);
+      } else if (e.key === 'Escape') {
+        closeDropdown();
+      }
+    });
+
+    input.addEventListener('blur', function () {
+      // Delay to allow click events on suggestions
+      setTimeout(closeDropdown, 150);
+    });
+
+    dropdown.addEventListener('mousedown', function (e) {
+      var item = e.target.closest('.autocomplete-suggestion');
+      if (item) {
+        e.preventDefault(); // prevent blur from firing before select
+        var idx = parseInt(item.getAttribute('data-index'), 10);
+        selectSuggestion(idx);
+      }
+    });
+  }());
+  </script>
 </body>
 </html>
 {{end}}