Fix paused timer auto-resuming on page navigation

applySnapshot was always setting isPaused = false and blockStartedAt =
Date.now() regardless of the actual timer state, causing a paused block
to appear running whenever the dashboard was reloaded.

- Add is_paused field to DashboardSnapshot schema
- Dashboard endpoint derives is_paused by checking whether the last
  start/resume/pause event for the current block is a pause
- applySnapshot now reads is_paused from the snapshot instead of
  resetting to false, and only sets blockStartedAt when the block is
  actually running (not paused)

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

backend/app/schemas/session.py

@@ -43,5 +43,6 @@ class DashboardSnapshot(BaseModel):
     blocks: list[ScheduleBlockOut] = []
     completed_block_ids: list[int] = []
     block_elapsed_seconds: int = 0  # seconds already elapsed for the current block
+    is_paused: bool = False          # whether the current block's timer is paused
     day_start_time: time | None = None
     day_end_time: time | None = None