Show expected vs actual pace/HR per lap in the Review Queue

Activities recorded from a structured Garmin workout carry a workoutId;
when its steps line up 1:1 with the recorded laps, the per-step target
pace/HR zone is resolved (via a Karvonen lookup for named zones) and stored
on each lap. The Review Queue plots it against the actual per-lap pace/HR
so the user can eyeball whether a run matches its plan while sorting it.

backend/internal/garmin/mock/mock.go

@@ -14,6 +14,7 @@ type Client struct {
 	Activities         []garmin.Activity
 	Splits             map[int64]garmin.ActivitySplits
 	Details            map[int64]garmin.ActivityDetails
+	Workouts           map[int64]garmin.Workout
 	Err                error // if set, every call returns this error
 	authResultCursor   int
 	ClosedCalled       bool
@@ -72,6 +73,13 @@ func (c *Client) GetActivityDetails(ctx context.Context, activityID int64) (garm
 	return c.Details[activityID], nil
 }
 
+func (c *Client) GetWorkoutByID(ctx context.Context, workoutID int64) (garmin.Workout, error) {
+	if c.Err != nil {
+		return garmin.Workout{}, c.Err
+	}
+	return c.Workouts[workoutID], nil
+}
+
 func (c *Client) UpdateCredentials(email, password string) {
 	c.LastEmail = email
 	c.LastPassword = password