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/state.js

@@ -4,7 +4,7 @@
  * emitting change events so the UI stays in sync.
  */
 
-export class State {
+class State {
     constructor() {
         /** @type {Array<Object>} */
         this.profiles = [];
@@ -223,3 +223,6 @@ export class State {
         this.emit('secretStore:init', initialised);
     }
 }
+
+window.NexusTerminal = window.NexusTerminal || {};
+window.NexusTerminal.State = State;