feat: implement changes tracking for university documents

- Added a new data structure `ScheduleChanges` to represent changes when a university document is republished.
- Implemented methods to read and write changes to a local JSON file.
- Enhanced the `Repository` class to detect changes between the current and fresh timetable versions.
- Updated the `MainViewModel` to expose changes to the UI and handle marking changes as seen.
- Created UI components for displaying changes in both Home and Settings screens.
- Added support for changes in iOS views, including a new `ChangesCard` and `ChangesSheet`.
- Implemented logic to compare timetable versions and identify added, removed, and changed lessons.

ios/PSUSchedule/AppModel.swift

@@ -24,6 +24,8 @@ final class AppModel {
     private(set) var exams: ExamsResponse?
     private(set) var allNotes: [Note]
     private(set) var allMarks: [Mark]
+    /** What changed when the university republished a document, per document and group. */
+    private(set) var allChanges: [ScheduleChanges]
     private(set) var status = Status()
     /** Moves every half minute while the app is open: the clock "Идёт · ещё 20 мин" counts from. */
     private(set) var now = Date()
@@ -39,6 +41,7 @@ final class AppModel {
         exams = Storage.read(ExamsResponse.self, "exams.json")
         allNotes = Storage.read([Note].self, "notes.json") ?? []
         allMarks = Storage.read([Mark].self, "attendance.json") ?? []
+        allChanges = Storage.read([ScheduleChanges].self, "changes.json") ?? []
         Task { [weak self] in
             while !Task.isCancelled {
                 try? await Task.sleep(for: .seconds(30))
@@ -62,6 +65,21 @@ final class AppModel {
         t.slots(on: date, subgroup: subgroup, hidden: hidden)
     }
 
+    /** The open group's last republished document: what changed for it. */
+    var changes: ScheduleChanges? {
+        guard let t = timetable else { return nil }
+        return allChanges.first { $0.document == t.document.id && $0.group == t.group }
+    }
+
+    /** The overview's card was opened or closed: the record stays for "Последние изменения". */
+    func markChangesSeen(_ record: ScheduleChanges) {
+        guard !record.seen else { return }
+        for i in allChanges.indices where allChanges[i].document == record.document && allChanges[i].group == record.group {
+            allChanges[i].seen = true
+        }
+        Storage.write(allChanges, "changes.json")
+    }
+
     /** Upcoming exams of the open group, soonest first; the cache of another group is ignored. */
     var upcomingExams: [Exam] {
         guard let t = timetable, let e = exams, e.group == t.group else { return [] }
@@ -110,6 +128,11 @@ final class AppModel {
         status.loading = true
         do {
             let fresh = try await block()
+            // Also in a background refresh: the card waits for the student on the overview.
+            if let record = ScheduleChanges.between(timetable, fresh) {
+                allChanges = Array(([record] + allChanges.filter { $0.document != record.document || $0.group != record.group }).prefix(12))
+                Storage.write(allChanges, "changes.json")
+            }
             timetable = fresh
             Storage.write(fresh, "timetable.json")
             lastFailed = nil