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/timetable.js

@@ -31,6 +31,10 @@ function lessonOut(lesson, group, people, date, status, calendar, week = null) {
     condition: lesson.condition,
     variant: lesson.variant,
     subgroup: lesson.subgroups.find((s) => s.group === group)?.label || null,
+    // "Немецкий язык" when part of the group studies German and part French (see `choices`).
+    option: lesson.option || null,
+    // "foreign": for foreign students only (Russian as a foreign language); "local": for the others.
+    audience: lesson.audience || null,
     groups: lesson.groups,
     uncertain: status === "maybe",
     raw: lesson.raw,
@@ -159,6 +163,10 @@ export function timetable({ file, schedule, calendar, group, from, days, range,
         ...extra.map((e) => e.lesson.subgroups?.find((s) => s.group === group)?.label).filter(Boolean),
       ]),
     ].sort((a, b) => a.localeCompare(b, "ru", { numeric: true })),
+    // What parts of the group choose between: [["Немецкий язык", "Французский язык"]].
+    choices: (schedule.choices || []).filter((c) => c.groups.includes(group)).map((c) => c.options),
+    // Some lessons are for foreign students only, or for the others: worth asking which one you are.
+    audience: lessons.some((l) => l.audience),
     bells: calendar?.bells || [],
     from,
     days: out,