refactor: restructure web application with modular JavaScript and remove legacy code

- Deleted the `app.js` file and migrated its functionality to a modular structure with multiple JavaScript files for better organization and maintainability.
- Updated `index.html` to reference the new `main.js` module, ensuring proper loading of the application.
- Introduced new utility modules for API requests, authentication, calendar handling, and DOM manipulation to enhance code clarity and separation of concerns.
- Enhanced CSS styles for improved layout and theming consistency across the application.
- Added comprehensive comments and documentation to new modules to facilitate future development and understanding.

webapp/js/utils.js

@@ -0,0 +1,14 @@
+/**
+ * Common utilities.
+ */
+
+/**
+ * Escape string for safe use in HTML (text content / attributes).
+ * @param {string} s - Raw string
+ * @returns {string} HTML-escaped string
+ */
+export function escapeHtml(s) {
+  const div = document.createElement("div");
+  div.textContent = s;
+  return div.innerHTML;
+}