Fix dashboard 500 error and stale activeChild token after regen

- timer.py: MySQL returns timezone-naive datetimes; wrap occurred_at
  with _utc() before arithmetic against datetime.now(timezone.utc) to
  avoid TypeError on compute_block_elapsed / compute_break_elapsed.
  This was causing a 500 on GET /api/dashboard/{token} for any child
  with an active session, preventing schedule templates from loading.

- children.js: after regenerateToken, also update activeChild so it
  doesn't hold a stale tv_token reference. Also sync activeChild on
  fetchChildren re-fetch so navigation after token regen picks up the
  new token immediately.

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

backend/app/utils/timer.py

@@ -7,6 +7,11 @@ from sqlalchemy import select
 from app.models.session import TimerEvent
 
 
+def _utc(dt: datetime) -> datetime:
+    """Return dt as UTC-aware, tagging naive datetimes (from MySQL) as UTC."""
+    return dt.replace(tzinfo=timezone.utc) if dt.tzinfo is None else dt
+
+
 async def compute_block_elapsed(
     db: AsyncSession, session_id: int, block_id: int
 ) -> tuple[int, bool]:
@@ -33,9 +38,9 @@ async def compute_block_elapsed(
             elapsed = 0.0
             last_start = None
         elif e.event_type in ("start", "resume"):
-            last_start = e.occurred_at
+            last_start = _utc(e.occurred_at)
         elif e.event_type == "pause" and last_start:
-            elapsed += (e.occurred_at - last_start).total_seconds()
+            elapsed += (_utc(e.occurred_at) - last_start).total_seconds()
             last_start = None
     running = last_start is not None
     if running:
@@ -67,11 +72,11 @@ async def compute_break_elapsed(
     for e in tick_events:
         if e.event_type == "break_reset":
             elapsed = 0.0
-            last_start = e.occurred_at
+            last_start = _utc(e.occurred_at)
         elif e.event_type in ("break_start", "break_resume"):
-            last_start = e.occurred_at
+            last_start = _utc(e.occurred_at)
         elif e.event_type == "break_pause" and last_start:
-            elapsed += (e.occurred_at - last_start).total_seconds()
+            elapsed += (_utc(e.occurred_at) - last_start).total_seconds()
             last_start = None
     running = last_start is not None
     if running: