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/auth/authStorage.ts

@@ -0,0 +1,22 @@
+import type { AuthState } from "../../api";
+
+const authStorageKey = "train-watcher-auth";
+
+export function loadAuth(): AuthState | null {
+  const raw = localStorage.getItem(authStorageKey);
+  if (!raw) return null;
+  try {
+    return JSON.parse(raw) as AuthState;
+  } catch {
+    localStorage.removeItem(authStorageKey);
+    return null;
+  }
+}
+
+export function saveAuth(auth: AuthState) {
+  localStorage.setItem(authStorageKey, JSON.stringify(auth));
+}
+
+export function clearAuth() {
+  localStorage.removeItem(authStorageKey);
+}