feat: implement Iteration 0 foundation (backend + Flutter client)

Backend (Go):
- Project structure with chi router, pgxpool, goose migrations
- JWT auth (access/refresh tokens) with Firebase token verification
- NoopTokenVerifier for local dev without Firebase credentials
- PostgreSQL user repository with atomic profile updates (transactions)
- Mifflin-St Jeor calorie calculation based on profile data
- REST API: POST /auth/login, /auth/refresh, /auth/logout, GET/PUT /profile, GET /health
- Middleware: auth, CORS (localhost wildcard), logging, recovery, request_id
- Unit tests (51 passing) and integration tests (testcontainers)
- Docker Compose setup with postgres healthcheck and graceful shutdown

Flutter client:
- Riverpod state management with GoRouter navigation
- Firebase Auth (email/password + Google sign-in with web popup support)
- Platform-aware API URLs (web/Android/iOS)
- Dio HTTP client with JWT auth interceptor and concurrent refresh handling
- Secure token storage
- Screens: Login, Register, Home (tabs: Menu, Recipes, Products, Profile)
- Unit tests (17 passing)

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

backend/internal/user/calories.go

@@ -0,0 +1,51 @@
+package user
+
+import "math"
+
+// Activity level multipliers (Mifflin-St Jeor).
+var activityMultiplier = map[string]float64{
+	"low":      1.375,
+	"moderate": 1.55,
+	"high":     1.725,
+}
+
+// Goal adjustments in kcal.
+var goalAdjustment = map[string]float64{
+	"lose":     -500,
+	"maintain": 0,
+	"gain":     300,
+}
+
+// CalculateDailyCalories computes the daily calorie target using the
+// Mifflin-St Jeor equation. Returns nil if any required parameter is missing.
+func CalculateDailyCalories(heightCM *int, weightKG *float64, age *int, gender, activity, goal *string) *int {
+	if heightCM == nil || weightKG == nil || age == nil || gender == nil || activity == nil || goal == nil {
+		return nil
+	}
+
+	// BMR = 10 * weight(kg) + 6.25 * height(cm) - 5 * age
+	bmr := 10**weightKG + 6.25*float64(*heightCM) - 5*float64(*age)
+
+	switch *gender {
+	case "male":
+		bmr += 5
+	case "female":
+		bmr -= 161
+	default:
+		return nil
+	}
+
+	mult, ok := activityMultiplier[*activity]
+	if !ok {
+		return nil
+	}
+
+	adj, ok := goalAdjustment[*goal]
+	if !ok {
+		return nil
+	}
+
+	tdee := bmr * mult
+	result := int(math.Round(tdee + adj))
+	return &result
+}