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/ui.js

@@ -0,0 +1,61 @@
+/**
+ * Screen states: access denied, error, nav enabled.
+ */
+
+import {
+  calendarEl,
+  dutyListEl,
+  loadingEl,
+  errorEl,
+  accessDeniedEl,
+  headerEl,
+  weekdaysEl,
+  prevBtn,
+  nextBtn
+} from "./dom.js";
+
+/**
+ * Show access-denied view and hide calendar/list/loading/error.
+ * @param {string} [serverDetail] - message from API 403 detail (unused; kept for callers)
+ */
+export function showAccessDenied(serverDetail) {
+  if (headerEl) headerEl.hidden = true;
+  if (weekdaysEl) weekdaysEl.hidden = true;
+  if (calendarEl) calendarEl.hidden = true;
+  if (dutyListEl) dutyListEl.hidden = true;
+  if (loadingEl) loadingEl.classList.add("hidden");
+  if (errorEl) errorEl.hidden = true;
+  if (accessDeniedEl) accessDeniedEl.hidden = false;
+}
+
+/**
+ * Hide access-denied and show calendar/list/header/weekdays.
+ */
+export function hideAccessDenied() {
+  if (accessDeniedEl) accessDeniedEl.hidden = true;
+  if (headerEl) headerEl.hidden = false;
+  if (weekdaysEl) weekdaysEl.hidden = false;
+  if (calendarEl) calendarEl.hidden = false;
+  if (dutyListEl) dutyListEl.hidden = false;
+}
+
+/**
+ * Show error message and hide loading.
+ * @param {string} msg - Error text
+ */
+export function showError(msg) {
+  if (errorEl) {
+    errorEl.textContent = msg;
+    errorEl.hidden = false;
+  }
+  if (loadingEl) loadingEl.classList.add("hidden");
+}
+
+/**
+ * Enable or disable prev/next month buttons.
+ * @param {boolean} enabled
+ */
+export function setNavEnabled(enabled) {
+  if (prevBtn) prevBtn.disabled = !enabled;
+  if (nextBtn) nextBtn.disabled = !enabled;
+}