feat: 优化线下收款日历交互与选中态

suixinkanTests/OfflineCollectionFeatureTests.swift

@@ -87,6 +87,35 @@ final class OfflineCollectionCalendarTests: XCTestCase {
         state.movePage(40)
         XCTAssertEqual(OfflineCollectionDate.businessDate(for: state.selectedDate), "2026-08-25")
     }
+
+    func testCalendarPageNavigationKeepsPositionAndUsesTodayAtFutureBoundary() throws {
+        let maximum = try XCTUnwrap(OfflineCollectionDate.date(from: "2026-08-26"))
+        let friday = try XCTUnwrap(OfflineCollectionDate.date(from: "2026-08-21"))
+        var week = OfflineCollectionCalendarState(selectedDate: friday, maximumDate: maximum)
+        XCTAssertTrue(week.canMovePage(1))
+        week.movePage(1)
+        XCTAssertEqual(OfflineCollectionDate.businessDate(for: week.selectedDate), "2026-08-26")
+        XCTAssertFalse(week.canMovePage(1))
+
+        let tuesday = try XCTUnwrap(OfflineCollectionDate.date(from: "2026-08-25"))
+        week = OfflineCollectionCalendarState(selectedDate: tuesday, maximumDate: maximum)
+        week.movePage(-1)
+        XCTAssertEqual(OfflineCollectionDate.businessDate(for: week.selectedDate), "2026-08-18")
+
+        let march = try XCTUnwrap(OfflineCollectionDate.date(from: "2025-03-31"))
+        var month = OfflineCollectionCalendarState(selectedDate: march, maximumDate: maximum, mode: .month)
+        month.movePage(-1)
+        XCTAssertEqual(OfflineCollectionDate.businessDate(for: month.selectedDate), "2025-02-28")
+    }
+
+    func testSelectedWeekRowIndexAndModeTogglePreserveDate() throws {
+        let selected = try XCTUnwrap(OfflineCollectionDate.date(from: "2026-08-12"))
+        var state = OfflineCollectionCalendarState(selectedDate: selected, maximumDate: selected)
+        XCTAssertEqual(state.selectedWeekRowIndex, 2)
+        state.toggleMode()
+        XCTAssertEqual(OfflineCollectionDate.businessDate(for: state.selectedDate), "2026-08-12")
+        XCTAssertEqual(state.selectedWeekRowIndex, 2)
+    }
 }
 
 /// 线下收款 ViewModel 的并发与幂等行为测试。