Update wire protocol to version 6 and enhance timetable functionality

- Bumped the wire protocol version to 6, reflecting changes in the communication structure.
- Expanded the timetable API with new endpoints for fetching and managing lesson schedules, including `GET /api/schools/{id}/timetable` and `POST /api/schools/{id}/timetable/pin`.
- Updated the protocol documentation to include detailed descriptions of the new timetable features and message structures.
- Enhanced the client-side implementation to support the new timetable functionalities, including lesson pinning and unpinning.
- Revised server-side logic to handle timetable operations and ensure proper integration with existing school management features.
- Added tests to validate the new timetable functionalities and ensure robustness in handling lesson data.

src/HSchool.Content/SchoolDay.cs

@@ -102,6 +102,10 @@ public static class SchoolDay
         return mondayBased < weekDays;
     }
 
+    /// <summary>Monday = 0 … Sunday = 6, same numbering the timetable uses.</summary>
+    public static int WeekdayIndex(DateTime time) =>
+        ((int)DateTime.SpecifyKind(time, DateTimeKind.Utc).DayOfWeek + 6) % 7;
+
     private static bool IsHoliday(DefCatalog catalog, DateTime time)
     {
         foreach (var holiday in catalog.Holidays.Values)