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/entity.go

@@ -33,7 +33,7 @@ type UpdateProfileRequest struct {
 	Activity      *string          `json:"activity"`
 	Goal          *string          `json:"goal"`
 	Preferences   *json.RawMessage `json:"preferences"`
-	DailyCalories *int             `json:"-"` // internal, set by service
+	DailyCalories *int             `json:"daily_calories,omitempty"`
 }
 
 // HasBodyParams returns true if any body parameter is being updated