Add break time feature to schedule blocks

- Admin: per-block "Break Time" checkbox + duration (min) setting; new
  Break Activities section (global list, same pattern as Morning Routine)
- Dashboard: break timer section appears on blocks with break enabled;
  Start/Pause/Resume/Reset controls work independently of the main timer
- TV: left column switches to amber break badge + countdown during break;
  center column shows configurable Break Activities list
- Backend: break_time_enabled/break_time_minutes columns on schedule_blocks
  (auto-migrated on startup); break_activity_items table + CRUD router;
  break timer events (break_start/pause/resume/reset) stored as TimerEvents
  and broadcast via WebSocket; break_activities included in dashboard
  snapshot and session_update broadcast

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

backend/app/schemas/schedule.py

@@ -11,6 +11,8 @@ class ScheduleBlockCreate(BaseModel):
     label: str | None = None
     notes: str | None = None
     order_index: int = 0
+    break_time_enabled: bool = False
+    break_time_minutes: int | None = None
 
 
 class ScheduleBlockUpdate(BaseModel):
@@ -21,6 +23,8 @@ class ScheduleBlockUpdate(BaseModel):
     label: str | None = None
     notes: str | None = None
     order_index: int | None = None
+    break_time_enabled: bool | None = None
+    break_time_minutes: int | None = None
 
 
 class ScheduleBlockOut(BaseModel):
@@ -33,6 +37,8 @@ class ScheduleBlockOut(BaseModel):
     label: str | None
     notes: str | None
     order_index: int
+    break_time_enabled: bool
+    break_time_minutes: int | None
 
     model_config = {"from_attributes": True}