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/constants.js

@@ -0,0 +1,20 @@
+/**
+ * Application constants and static labels.
+ */
+
+export const FETCH_TIMEOUT_MS = 15000;
+export const RETRY_DELAY_MS = 800;
+export const RETRY_AFTER_ACCESS_DENIED_MS = 1200;
+
+export const THEME_BG = { dark: "#17212b", light: "#d5d6db" };
+
+export const MONTHS = [
+  "Январь", "Февраль", "Март", "Апрель", "Май", "Июнь",
+  "Июль", "Август", "Сентябрь", "Октябрь", "Ноябрь", "Декабрь"
+];
+
+export const EVENT_TYPE_LABELS = {
+  duty: "Дежурство",
+  unavailable: "Недоступен",
+  vacation: "Отпуск"
+};