feat: 优化线下收款日历交互与选中态

suixinkan/Features/OfflineCollection/Models/OfflineCollectionCalendarModel.swift

@@ -56,6 +56,28 @@ struct OfflineCollectionCalendarState: Sendable, Equatable {
         return selectedDate
     }
 
+    /// 判断指定方向是否存在不晚于服务端今日的周或月页面。
+    func canMovePage(_ offset: Int) -> Bool {
+        guard offset != 0 else { return false }
+        if offset < 0 { return true }
+        switch mode {
+        case .week:
+            guard let candidate = calendar.date(byAdding: .day, value: offset * 7, to: selectedDate) else { return false }
+            let weekday = calendar.component(.weekday, from: candidate)
+            let daysFromMonday = (weekday + 5) % 7
+            let targetMonday = calendar.date(byAdding: .day, value: -daysFromMonday, to: candidate) ?? candidate
+            return calendar.startOfDay(for: targetMonday) <= maximumDate
+        case .month:
+            let current = calendar.dateComponents([.year, .month], from: selectedDate)
+            guard let monthStart = calendar.date(from: current),
+                  let targetMonth = calendar.date(byAdding: .month, value: offset, to: monthStart) else { return false }
+            let maximumMonth = calendar.date(
+                from: calendar.dateComponents([.year, .month], from: maximumDate)
+            ) ?? maximumDate
+            return targetMonth <= maximumMonth
+        }
+    }
+
     /// 当前周的周一至周日。
     var weekDates: [Date] {
         let weekday = calendar.component(.weekday, from: selectedDate)
@@ -74,6 +96,12 @@ struct OfflineCollectionCalendarState: Sendable, Equatable {
         return (0 ..< 42).compactMap { calendar.date(byAdding: .day, value: $0, to: start) }
     }
 
+    /// 当前选中日期所在周在固定六行月历中的零基行号。
+    var selectedWeekRowIndex: Int {
+        let index = monthDates.firstIndex { calendar.isDate($0, inSameDayAs: selectedDate) } ?? 0
+        return index / 7
+    }
+
     /// 当前选中月份标题。
     var monthTitle: String {
         let parts = calendar.dateComponents([.year, .month], from: selectedDate)