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/index.html

@@ -3,11 +3,12 @@
 <div class="index">
   <div class="title"><h1>kafka</h1></div>
   <div id="search">
-    <form method="GET" action="/search" role="search">
+    <form method="GET" action="/search" role="search" id="search-form">
       <input type="text" name="q" id="q" placeholder="Search…" autocomplete="off" autofocus
         hx-get="/search" hx-target="#results" hx-trigger="keyup changed delay:500ms" hx-include="this">
       <button type="submit">Search</button>
     </form>
+    <div id="autocomplete-dropdown"></div>
   </div>
 </div>
 <div id="results"></div>