Add Reset button and move End Day to right side of actions bar

Reset clears the current block's elapsed time to zero and immediately
starts the timer. A shared compute_block_elapsed() utility (utils/timer.py)
handles the elapsed calculation in both the sessions and dashboard routers,
and correctly treats "reset" events as zero-elapsed restart markers so
page reloads after a reset show accurate times.

Layout: Start/Pause/Resume/Reset are grouped on the left; End Day sits
on the right via justify-content: space-between.

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

backend/app/routers/dashboard.py

@@ -2,7 +2,7 @@
 Public dashboard endpoint — no authentication required.
 Used by the TV view to get the initial session snapshot before WebSocket connects.
 """
-from datetime import date, datetime
+from datetime import date
 
 from fastapi import APIRouter, Depends, HTTPException
 from sqlalchemy.ext.asyncio import AsyncSession
@@ -16,6 +16,7 @@ from app.models.schedule import ScheduleBlock, ScheduleTemplate
 from app.models.subject import Subject  # noqa: F401 — needed for selectinload chain
 from app.models.session import DailySession, TimerEvent
 from app.schemas.session import DashboardSnapshot
+from app.utils.timer import compute_block_elapsed
 
 router = APIRouter(prefix="/api/dashboard", tags=["dashboard"])
 
@@ -74,29 +75,9 @@ async def get_dashboard(child_id: int, db: AsyncSession = Depends(get_db)):
     # Compute elapsed seconds and paused state for the current block from timer_events
     is_paused = False
     if session and session.current_block_id:
-        tick_result = await db.execute(
-            select(TimerEvent)
-            .where(
-                TimerEvent.session_id == session.id,
-                TimerEvent.block_id == session.current_block_id,
-                TimerEvent.event_type.in_(["start", "resume", "pause"]),
-            )
-            .order_by(TimerEvent.occurred_at)
+        block_elapsed_seconds, is_paused = await compute_block_elapsed(
+            db, session.id, session.current_block_id
         )
-        tick_events = tick_result.scalars().all()
-        last_start = None
-        elapsed = 0.0
-        for e in tick_events:
-            if e.event_type in ("start", "resume"):
-                last_start = e.occurred_at
-            elif e.event_type == "pause" and last_start:
-                elapsed += (e.occurred_at - last_start).total_seconds()
-                last_start = None
-        if last_start:
-            elapsed += (datetime.utcnow() - last_start).total_seconds()
-        block_elapsed_seconds = int(elapsed)
-        # Paused if the last tick event was a pause (last_start is None but events exist)
-        is_paused = bool(tick_events) and tick_events[-1].event_type == "pause"
 
     routine_result = await db.execute(
         select(MorningRoutineItem)