feat(js): load app.js using JavaScript module

- The JS bundle has its own isolated scope
- There is no need to use IIFEs anymore (Immediately Invoked Function Expressions)
- Modules are executed after the HTML document is fully parsed, similar to `defer` attribute
- There is no need to use `DOMContentLoaded` anymore
- Module scripts inherently run in strict mode (no need to define `use strict` anymore)

internal/template/templates/common/layout.html

@@ -48,7 +48,7 @@
         <meta http-equiv="Content-Security-Policy" content="default-src 'self'; img-src * data:; media-src *; frame-src *; require-trusted-types-for 'script'; trusted-types ttpolicy;">
     {{ end }}
 
-    <script src="{{ route "javascript" "name" "app" "checksum" .app_js_checksum }}" defer></script>
+    <script src="{{ route "javascript" "name" "app" "checksum" .app_js_checksum }}" type="module"></script>
     <script src="{{ route "javascript" "name" "service-worker" "checksum" .sw_js_checksum }}" defer id="service-worker-script"></script>
 </head>
 <body