Sort schedule blocks by start time instead of order_index

Backend queries and model relationship now order by time_start.
Frontend also sorts blocks client-side for reliability across all views.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/routers/sessions.py

@@ -28,7 +28,7 @@ async def _broadcast_session(db: AsyncSession, session: DailySession) -> None:
             select(ScheduleBlock)
             .where(ScheduleBlock.template_id == session.template_id)
             .options(selectinload(ScheduleBlock.subject).selectinload(Subject.options))
-            .order_by(ScheduleBlock.order_index)
+            .order_by(ScheduleBlock.time_start)
         )
         blocks = [
             {