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/ui/static/js/app.js

@@ -489,6 +489,9 @@ function appendIconLabel(element, labelTextContent) {
 function toggleEntryStatus(element, toasting) {
     const entryID = parseInt(element.dataset.id, 10);
     const link = element.querySelector(":is(a, button)[data-toggle-status]");
+    if (!link) {
+        return;
+    }
 
     const currentStatus = link.dataset.value;
     const newStatus = currentStatus === "read" ? "unread" : "read";