Add date and time handling, models, schedule management, storage, and widget support

- Introduced `Dates.swift` for handling date and time in the university's timezone.
- Created `Models.swift` to define data structures for timetable, lessons, and directory search results.
- Implemented `Schedule.swift` for managing lesson slots and timetable functionalities.
- Developed `Storage.swift` for user settings and data persistence.
- Added `Widgets.swift` to create widgets displaying lessons and upcoming classes.

ios/Shared/Storage.swift

@@ -0,0 +1,109 @@
+import Foundation
+
+enum ThemeMode: String, Codable, CaseIterable {
+    case system, light, dark
+}
+
+enum PlansLayout: String, Codable, CaseIterable {
+    case agenda, list, board
+}
+
+/** How the Plans screen lays out tasks, as in School Planner's plan settings. */
+struct PlansPrefs: Codable, Hashable {
+    var layout: PlansLayout = .agenda
+    /** The day's lessons above its plans; off by default, so plans come first. */
+    var lessons: Bool = false
+    /** Ticked-off tasks stay in the list. */
+    var done: Bool = true
+}
+
+/** What lesson cards, the week grid and the overview show; everything is on by default. */
+struct Display: Codable, Hashable {
+    var colors = true
+    var teachers = true
+    var rooms = true
+    var pairs = true
+    var tags = true
+    var notes = true
+}
+
+/** Everything the student chose; stays on the device, shared with the widgets. */
+struct UserSettings: Codable, Hashable {
+    var current: SavedGroup?
+    /** The chosen subgroup per group ("docId|group"). */
+    var subgroups: [String: String] = [:]
+    var theme: ThemeMode = .system
+    var reminders = false
+    var reminderMinutes = 10
+    var recent: [SavedGroup] = []
+    var display = Display()
+    /** Hidden lessons per group, as Lesson.hideKey values. */
+    var hiddenByGroup: [String: [String]] = [:]
+    var plans = PlansPrefs()
+
+    var subgroup: String? { current.flatMap { subgroups[$0.key] } }
+    var hidden: Set<String> { Set(current.flatMap { hiddenByGroup[$0.key] } ?? []) }
+}
+
+// Every field may be missing in an older save.
+extension UserSettings {
+    init(from decoder: Decoder) throws {
+        let c = try decoder.container(keyedBy: CodingKeys.self)
+        current = try c.decodeIfPresent(SavedGroup.self, forKey: .current)
+        subgroups = try c.decodeIfPresent([String: String].self, forKey: .subgroups) ?? [:]
+        theme = try c.decodeIfPresent(ThemeMode.self, forKey: .theme) ?? .system
+        reminders = try c.decodeIfPresent(Bool.self, forKey: .reminders) ?? false
+        reminderMinutes = try c.decodeIfPresent(Int.self, forKey: .reminderMinutes) ?? 10
+        recent = try c.decodeIfPresent([SavedGroup].self, forKey: .recent) ?? []
+        display = try c.decodeIfPresent(Display.self, forKey: .display) ?? Display()
+        hiddenByGroup = try c.decodeIfPresent([String: [String]].self, forKey: .hiddenByGroup) ?? [:]
+        plans = try c.decodeIfPresent(PlansPrefs.self, forKey: .plans) ?? PlansPrefs()
+    }
+}
+
+/**
+ * Files and settings in the app group container, so the widgets read the same timetable,
+ * notes and choices as the app. Everything stays on the device.
+ */
+enum Storage {
+    static let group = "group.org.xtimms.psu.schedule"
+
+    static var directory: URL {
+        if let url = FileManager.default.containerURL(forSecurityApplicationGroupIdentifier: group) { return url }
+        // Without the app group (an unsigned build) the app still works on its own.
+        let url = FileManager.default.urls(for: .applicationSupportDirectory, in: .userDomainMask)[0]
+        try? FileManager.default.createDirectory(at: url, withIntermediateDirectories: true)
+        return url
+    }
+
+    static var defaults: UserDefaults { UserDefaults(suiteName: group) ?? .standard }
+
+    static let encoder = JSONEncoder()
+    static let decoder = JSONDecoder()
+
+    static func read<T: Decodable>(_ type: T.Type, _ name: String) -> T? {
+        guard let data = try? Data(contentsOf: directory.appendingPathComponent(name)) else { return nil }
+        return try? decoder.decode(type, from: data)
+    }
+
+    static func write<T: Encodable>(_ value: T, _ name: String) {
+        guard let data = try? encoder.encode(value) else { return }
+        try? data.write(to: directory.appendingPathComponent(name), options: .atomic)
+    }
+
+    static func remove(_ name: String) {
+        try? FileManager.default.removeItem(at: directory.appendingPathComponent(name))
+    }
+
+    static func loadSettings() -> UserSettings {
+        guard let data = defaults.data(forKey: "settings"),
+              let s = try? decoder.decode(UserSettings.self, from: data) else { return UserSettings() }
+        return s
+    }
+
+    static func saveSettings(_ s: UserSettings) {
+        if let data = try? encoder.encode(s) { defaults.set(data, forKey: "settings") }
+    }
+
+    static func loadTimetable() -> Timetable? { read(Timetable.self, "timetable.json") }
+}