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/utils/timer.py

@@ -0,0 +1,44 @@
+"""Shared timer-elapsed computation used by sessions and dashboard routers."""
+from datetime import datetime
+
+from sqlalchemy.ext.asyncio import AsyncSession
+from sqlalchemy import select
+
+from app.models.session import TimerEvent
+
+
+async def compute_block_elapsed(
+    db: AsyncSession, session_id: int, block_id: int
+) -> tuple[int, bool]:
+    """Return (elapsed_seconds, is_paused) for a block.
+
+    'reset' events are treated as zero-elapsed restart markers: any elapsed
+    time accumulated before a reset is discarded.
+    """
+    tick_result = await db.execute(
+        select(TimerEvent)
+        .where(
+            TimerEvent.session_id == session_id,
+            TimerEvent.block_id == block_id,
+            TimerEvent.event_type.in_(["start", "resume", "pause", "reset"]),
+        )
+        .order_by(TimerEvent.occurred_at)
+    )
+    tick_events = tick_result.scalars().all()
+
+    elapsed = 0.0
+    last_start = None
+    for e in tick_events:
+        if e.event_type == "reset":
+            elapsed = 0.0
+            last_start = e.occurred_at
+        elif 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()
+
+    is_paused = bool(tick_events) and tick_events[-1].event_type == "pause"
+    return int(elapsed), is_paused