feat: add onboarding flow with visual redesign

Introduce 6-step onboarding screen (Goal → Gender → DOB → Height+Weight
→ Activity → Calories) with per-step accent colors, hero illustration
area (concentric circles + icon), and white card content panel.

Backend user entity and service updated to support onboarding fields
(goal, activity, height, weight, DOB, dailyCalories). Router guards
unauthenticated and onboarding-incomplete users. Profile service and
screen updated to expose language and onboarding preferences.

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

backend/internal/domain/user/service.go

@@ -65,7 +65,12 @@ func (s *Service) UpdateProfile(ctx context.Context, userID string, req UpdatePr
 		goal = req.Goal
 	}
 
-	calories := CalculateDailyCalories(height, weight, age, gender, activity, goal)
+	var calories *int
+	if req.DailyCalories != nil {
+		calories = req.DailyCalories
+	} else {
+		calories = CalculateDailyCalories(height, weight, age, gender, activity, goal)
+	}
 
 	var calReq *UpdateProfileRequest
 	if calories != nil {