feat: replace inline chat-input with custom element and migrate autocomplete logic

Refactor the chat input area in web.html to use the new ChatInputElement custom component instead of inline HTML. Move autocomplete handler from a function-based getInputField() to directly attaching to the chat-input element's autoCompletions property. Remove the legacy initInputField function and its associated keydown/input event listeners, consolidating all input behavior into the component. Add the chat-input.js module script to app.html to ensure the custom element is registered globally.

src/snek/templates/app.html

@@ -18,6 +18,7 @@
   <script src="/file-manager.js" type="module"></script>
   <script src="/user-list.js"></script>
   <script src="/message-list.js" type="module"></script>
+  <script src="/chat-input.js" type="module"></script>
   <link rel="stylesheet" href="/user-list.css">
 
   <link rel="stylesheet" href="/base.css">