feat: Add skeleton loading UI components for various screens

- Implemented `Skeleton.kt` for loading placeholders in the app, including lesson cards, schedule, overview, and subjects.
- Introduced `Undo.kt` to manage undo actions with a snackbar interface.
- Created `WeekWidget.kt` for a widget displaying the week's schedule, including dynamic updates based on the current timetable.
- Added various splash animations in the `animator` directory for a smooth launch experience.
- Designed drawable resources for the widget and splash screen, including icons for city and room moves.
- Defined XML resources for widget configuration and photo paths.

server/index.js

@@ -80,7 +80,9 @@ app.use("/api", (_req, res, next) => {
 // 12 — a JSON log line per request;
 // 13 — the dashboard at /status (password), /api/health only says alive, ready and versions;
 // 14 — a lesson's `place` and a day's `places` ("Занятия в Полоцке") in /api/timetable.
-const API_VERSION = 14;
+// 15 — a lesson's `option` and the group's `choices` (German or French for parts of a group),
+//      a lesson's `audience` (foreign students or the others) and the group's `audience` flag.
+const API_VERSION = 15;
 
 /**
  * What the server still waits for before it answers from memory: the catalog, the week table