feat(diary): editing of previous and current finished days

lib/features/diary/presentation/diary_edit_access.dart

@@ -0,0 +1,38 @@
+/// Finished and last-day-finished diaries stay editable on the calendar day
+/// itself and on the next calendar day. Older finished days stay read-only.
+const int diaryFinishedEditWindowInDays = 1;
+
+bool canEditDiaryDay({
+  required bool isConsultantView,
+  required bool isDiaryCommented,
+  required bool isDiaryFinished,
+  required int closed,
+  required String diaryDate,
+  DateTime? now,
+}) {
+  if (isConsultantView || isDiaryCommented || closed != 0) {
+    return false;
+  }
+  if (!isDiaryFinished) {
+    return true;
+  }
+  final day = _calendarDate(diaryDate);
+  if (day == null) {
+    return false;
+  }
+  final clock = now ?? DateTime.now();
+  final today = DateTime(clock.year, clock.month, clock.day);
+  final oldestEditableDay = today.subtract(
+    const Duration(days: diaryFinishedEditWindowInDays),
+  );
+  return !day.isBefore(oldestEditableDay) && !day.isAfter(today);
+}
+
+DateTime? _calendarDate(String raw) {
+  final parsed = DateTime.tryParse(raw);
+  if (parsed == null) {
+    return null;
+  }
+  final local = parsed.toLocal();
+  return DateTime(local.year, local.month, local.day);
+}