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/terminal.js

@@ -6,7 +6,7 @@
 
 /* global Terminal, FitAddon, SearchAddon, WebLinksAddon */
 
-export class TerminalManager {
+class TerminalManager {
     /**
      * @param {import('./app.js').App} app
      */
@@ -284,3 +284,6 @@ export class TerminalManager {
         };
     }
 }
+
+window.NexusTerminal = window.NexusTerminal || {};
+window.NexusTerminal.TerminalManager = TerminalManager;