Convert ES modules to plain scripts using window.NexusTerminal namespace

Replace ES module import/export with a global namespace pattern to avoid
silent failures in the Wails v2 WKWebView which does not reliably support
<script type="module"> tags.

- Remove all import/export statements from JS files
- Register each module's exports on window.NexusTerminal
- Destructure dependencies from window.NexusTerminal at file top
- Load all scripts in dependency order in index.html as plain <script> tags

frontend/src/js/settings.js

@@ -4,7 +4,7 @@
  * terminal defaults, security, known-hosts management, and about info.
  */
 
-import { createElement, escapeHtml, formatTimestamp, $ } from './utils.js';
+const { createElement, escapeHtml, formatTimestamp, $ } = window.NexusTerminal;
 
 /** Settings tab definitions. */
 const TABS = [
@@ -15,7 +15,7 @@ const TABS = [
     { key: 'about', label: 'About' },
 ];
 
-export class Settings {
+class Settings {
     /**
      * @param {import('./app.js').App} app
      */
@@ -461,3 +461,6 @@ export class Settings {
         }
     }
 }
+
+window.NexusTerminal = window.NexusTerminal || {};
+window.NexusTerminal.Settings = Settings;