feat: Implement active workout flow with status management

- Added `status`, `total_sets`, and `total_volume` fields to the Workout model.
- Introduced `source_kind`, `title_snapshot`, and `image_s3_url_snapshot` fields to the WorkoutItem model.
- Created endpoints for managing active workouts, including finishing and discarding workouts.
- Updated workout creation to ensure only one active workout exists per user.
- Implemented batch addition of workout sets and updates to workout set details.
- Enhanced database schema with Alembic migrations to support new fields and constraints.
- Added validation to ensure at least one field is provided for workout set updates.
- Updated calorie estimation logic to reflect new workout set structure.

frontend/src/features/workout/EmptyWorkoutState.tsx

@@ -0,0 +1,10 @@
+export function EmptyWorkoutState({ onOpenDrawer }: { onOpenDrawer: () => void }) {
+  return (
+    <section className="empty-workout card">
+      <p className="eyebrow">Журнал пуст</p>
+      <h3>Добавь первое упражнение</h3>
+      <p>Каталог теперь открывается отдельным drawer, поэтому основной экран остается чистым журналом выполнения.</p>
+      <button className="primary" onClick={onOpenDrawer}>+ Добавить упражнение</button>
+    </section>
+  );
+}