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/dateUtils.js

@@ -0,0 +1,91 @@
+/**
+ * Date/time helpers for calendar and duty display.
+ */
+
+/**
+ * YYYY-MM-DD in local time (for calendar keys, "today", request range).
+ * @param {Date} d - Date
+ * @returns {string}
+ */
+export function localDateString(d) {
+  const y = d.getFullYear();
+  const m = String(d.getMonth() + 1).padStart(2, "0");
+  const day = String(d.getDate()).padStart(2, "0");
+  return y + "-" + m + "-" + day;
+}
+
+/**
+ * True if duty (start_at/end_at UTC) overlaps the local day YYYY-MM-DD.
+ * @param {object} d - Duty with start_at, end_at
+ * @param {string} dateKey - YYYY-MM-DD
+ * @returns {boolean}
+ */
+export function dutyOverlapsLocalDay(d, dateKey) {
+  const [y, m, day] = dateKey.split("-").map(Number);
+  const dayStart = new Date(y, m - 1, day, 0, 0, 0, 0);
+  const dayEnd = new Date(y, m - 1, day, 23, 59, 59, 999);
+  const start = new Date(d.start_at);
+  const end = new Date(d.end_at);
+  return end > dayStart && start < dayEnd;
+}
+
+/** @param {Date} d - Date */
+export function firstDayOfMonth(d) {
+  return new Date(d.getFullYear(), d.getMonth(), 1);
+}
+
+/** @param {Date} d - Date */
+export function lastDayOfMonth(d) {
+  return new Date(d.getFullYear(), d.getMonth() + 1, 0);
+}
+
+/** @param {Date} d - Date (returns Monday of that week) */
+export function getMonday(d) {
+  const day = d.getDay();
+  const diff = d.getDate() - day + (day === 0 ? -6 : 1);
+  return new Date(d.getFullYear(), d.getMonth(), diff);
+}
+
+/**
+ * Format UTC date from ISO string as DD.MM for display.
+ * @param {string} isoDateStr - ISO date string
+ * @returns {string} DD.MM
+ */
+export function formatDateKey(isoDateStr) {
+  const d = new Date(isoDateStr);
+  const day = String(d.getDate()).padStart(2, "0");
+  const month = String(d.getMonth() + 1).padStart(2, "0");
+  return day + "." + month;
+}
+
+/**
+ * Format YYYY-MM-DD as DD.MM for list header.
+ * @param {string} key - YYYY-MM-DD
+ * @returns {string} DD.MM
+ */
+export function dateKeyToDDMM(key) {
+  return key.slice(8, 10) + "." + key.slice(5, 7);
+}
+
+/**
+ * Format ISO date as HH:MM in local time.
+ * @param {string} isoStr - ISO date string
+ * @returns {string} HH:MM
+ */
+export function formatTimeLocal(isoStr) {
+  const d = new Date(isoStr);
+  return String(d.getHours()).padStart(2, "0") + ":" + String(d.getMinutes()).padStart(2, "0");
+}
+
+/**
+ * Format ISO string as HH:MM (local).
+ * @param {string} isoStr - ISO date string
+ * @returns {string} HH:MM or ""
+ */
+export function formatHHMM(isoStr) {
+  if (!isoStr) return "";
+  const d = new Date(isoStr);
+  const h = d.getHours();
+  const m = d.getMinutes();
+  return (h < 10 ? "0" : "") + h + ":" + (m < 10 ? "0" : "") + m;
+}