Add Day Started event to activity log with template name

- On session start, insert a session_start TimerEvent so the action
  appears in the activity log timeline
- Load DailySession.template via selectinload in the timeline query
  so the template name is available without extra round-trips
- _to_timeline_out maps the template name into block_label for
  session_start events, displaying as "Day started — Template Name"
- Add session_start to EVENT_META on the frontend (🏫 icon)
- Hide the Edit button for session_start entries since changing their
  event type to a block-level action doesn't make sense

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

backend/app/routers/sessions.py

@@ -126,6 +126,11 @@ async def start_session(
     db.add(session)
     await db.commit()
     await db.refresh(session)
+
+    # Record session start as a timer event so it appears in the activity log
+    db.add(TimerEvent(session_id=session.id, block_id=None, event_type="session_start"))
+    await db.commit()
+
     await _broadcast_session(db, session)
     return session