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/static/css/kafka.css

@@ -421,6 +421,63 @@ footer a:hover {
   display: block;
 }
 
+/* Autocomplete dropdown */
+#search {
+  position: relative;
+}
+
+#autocomplete-dropdown {
+  position: absolute;
+  top: 100%;
+  left: 0;
+  right: 0;
+  background: var(--color-base-background);
+  border: 1px solid var(--color-search-border);
+  border-top: none;
+  border-radius: 0 0 var(--radius) var(--radius);
+  box-shadow: 0 4px 6px rgba(0, 0, 0, 0.1);
+  z-index: 100;
+  max-height: 320px;
+  overflow-y: auto;
+  display: none;
+}
+
+#autocomplete-dropdown.open {
+  display: block;
+}
+
+.autocomplete-suggestion {
+  padding: 0.6rem 1rem;
+  cursor: pointer;
+  font-size: 0.95rem;
+  color: var(--color-base-font);
+  border-bottom: 1px solid var(--color-result-border);
+  transition: background 0.15s;
+}
+
+.autocomplete-suggestion:last-child {
+  border-bottom: none;
+}
+
+.autocomplete-suggestion:hover,
+.autocomplete-suggestion.active {
+  background: var(--color-header-background);
+}
+
+.autocomplete-suggestion mark {
+  background: none;
+  color: var(--color-link);
+  font-weight: 600;
+}
+
+.autocomplete-footer {
+  padding: 0.4rem 1rem;
+  font-size: 0.75rem;
+  color: var(--color-suggestion);
+  border-top: 1px solid var(--color-result-border);
+  background: var(--color-header-background);
+}
+
 /* Responsive */
 @media (max-width: 768px) {
   #results {