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/Shared/Changes.swift

@@ -0,0 +1,151 @@
+import Foundation
+
+// What changed for the group when the university republished its document, as the PWA reports it
+// (shared/lessons.js diffSchedules) and the Android client's Changes.kt: the new timetable is
+// compared with the copy on the device.
+
+/// A Word row of the timetable: a weekday's lesson, or a lesson on its own date.
+struct ChangedLesson: Codable, Hashable {
+    var day: Int
+    var date: String?
+    var lesson: Lesson
+}
+
+struct FieldChange: Codable, Hashable {
+    var label: String
+    var from: String
+    var to: String
+}
+
+struct LessonChange: Codable, Hashable {
+    var before: ChangedLesson
+    var after: ChangedLesson
+    var fields: [FieldChange]
+}
+
+struct ScheduleChanges: Codable, Hashable {
+    var document: String
+    var group: String
+    var name: String
+    /// When the change was found.
+    var at: Date
+    /// Dates of the two versions of the file, as the server sends them.
+    var from: String?
+    var to: String?
+    var added: [ChangedLesson] = []
+    var removed: [ChangedLesson] = []
+    var changed: [LessonChange] = []
+    /// The card on the overview was opened or closed.
+    var seen = false
+
+    var count: Int { added.count + removed.count + changed.count }
+
+    /**
+     * Compares a newer version of the same document with the copy on the device: only a
+     * republished file counts, not a new parser or another document of the group.
+     */
+    static func between(_ previous: Timetable?, _ fresh: Timetable, now: Date = Date()) -> ScheduleChanges? {
+        guard let previous, previous.document.id == fresh.document.id, previous.group == fresh.group,
+              let was = previous.document.modified, let then = fresh.document.modified, was != then
+        else { return nil }
+        let (added, removed, changed) = diff(previous, fresh)
+        let record = ScheduleChanges(
+            document: fresh.document.id, group: fresh.group, name: fresh.document.name, at: now,
+            from: was, to: then, added: added, removed: removed, changed: changed
+        )
+        return record.count > 0 ? record : nil
+    }
+}
+
+/// "3 изменения".
+func changesLabel(_ n: Int) -> String { "\(n) \(plural(n, "изменение", "изменения", "изменений"))" }
+
+/// The date of a file version as the server sends it ("Mon, 19 Oct 2026 08:00:00 GMT").
+func fileDateLabel(_ value: String?) -> String? {
+    guard let value else { return nil }
+    let f = DateFormatter()
+    f.locale = Locale(identifier: "en_US_POSIX")
+    f.dateFormat = "EEE, dd MMM yyyy HH:mm:ss zzz"
+    return f.date(from: value).map { CivilDate($0).longLabel }
+}
+
+/**
+ * The timetable's rows: the weekly Word rows, plus lessons printed with their own date (the
+ * server's key then starts with that date). Dates computed from week marks are not compared one
+ * by one, or one changed mark would read as a dozen changes.
+ */
+private func rows(_ t: Timetable) -> [ChangedLesson] {
+    let weekly = t.weekly.flatMap { d in d.lessons.map { ChangedLesson(day: d.day, date: nil, lesson: $0) } }
+    var seen = Set<String>()
+    let dated = t.days.flatMap { d in
+        d.lessons.filter { $0.key.range(of: #"^\d{4}-\d{2}-\d{2}\|"#, options: .regularExpression) != nil }
+            .map { ChangedLesson(day: d.weekday, date: d.date, lesson: $0) }
+    }.filter { seen.insert($0.lesson.key).inserted }
+    return weekly + dated
+}
+
+private func norm(_ s: String) -> String {
+    s.lowercased().split(whereSeparator: \.isWhitespace).joined(separator: " ")
+}
+
+/// Looser identity for pairing a lesson whose day, time or week mark changed.
+private func looseKey(_ l: Lesson) -> String {
+    [norm(l.subject), l.variant, l.subgroup ?? "", norm(l.type)].joined(separator: "|")
+}
+
+private func whenLabel(_ row: ChangedLesson) -> String {
+    row.date.flatMap { CivilDate($0) }.map { "\($0.weekdayName), \($0.longLabel)" } ?? dayName(row.day)
+}
+
+private func fields(_ a: ChangedLesson, _ b: ChangedLesson) -> [FieldChange] {
+    let pairs: [(String, String, String)] = [
+        ("День", whenLabel(a), whenLabel(b)),
+        ("Начало", a.lesson.start, b.lesson.start),
+        ("Окончание", a.lesson.end ?? "", b.lesson.end ?? ""),
+        ("Недели", a.lesson.condition, b.lesson.condition),
+        ("Вид занятия", a.lesson.type, b.lesson.type),
+        ("Преподаватель", a.lesson.teachers.joined(separator: ", "), b.lesson.teachers.joined(separator: ", ")),
+        ("Аудитория", a.lesson.rooms.joined(separator: ", "), b.lesson.rooms.joined(separator: ", ")),
+    ]
+    let shown = { (s: String) in s.trimmed.isEmpty ? "не указано" : s }
+    return pairs.filter { $0.1.trimmed != $0.2.trimmed }.map { FieldChange(label: $0.0, from: shown($0.1), to: shown($0.2)) }
+}
+
+private func order(_ a: ChangedLesson, _ b: ChangedLesson) -> Bool {
+    ((a.date ?? ""), a.day, a.lesson.start) < ((b.date ?? ""), b.day, b.lesson.start)
+}
+
+/// Added, removed and changed rows between two versions of the group's timetable.
+private func diff(_ before: Timetable, _ after: Timetable) -> ([ChangedLesson], [ChangedLesson], [LessonChange]) {
+    var old: [String: [ChangedLesson]] = [:]
+    var order0: [String] = []
+    for row in rows(before) {
+        if old[row.lesson.key] == nil { order0.append(row.lesson.key) }
+        old[row.lesson.key, default: []].append(row)
+    }
+    var changed: [LessonChange] = []
+    var unmatched: [ChangedLesson] = []
+    for row in rows(after) {
+        if var list = old[row.lesson.key], !list.isEmpty {
+            let was = list.removeFirst()
+            old[row.lesson.key] = list
+            let f = fields(was, row)
+            if !f.isEmpty { changed.append(LessonChange(before: was, after: row, fields: f)) }
+        } else {
+            unmatched.append(row)
+        }
+    }
+    var left = order0.flatMap { old[$0] ?? [] }
+    // Pair the rest by subject on the same day first, then by subject alone (moved to another day).
+    for sameDay in [true, false] {
+        for row in unmatched {
+            guard let index = left.firstIndex(where: {
+                looseKey($0.lesson) == looseKey(row.lesson) && (!sameDay || ($0.date ?? "\($0.day)") == (row.date ?? "\(row.day)"))
+            }) else { continue }
+            let was = left.remove(at: index)
+            if let i = unmatched.firstIndex(of: row) { unmatched.remove(at: i) }
+            changed.append(LessonChange(before: was, after: row, fields: fields(was, row)))
+        }
+    }
+    return (unmatched.sorted(by: order), left.sorted(by: order), changed.sorted { order($0.after, $1.after) })
+}