Remove timezone from User model and pass as request parameter

Timezone is now sent from the client on each request that needs it,
instead of storing it in the database. This simplifies the model and
allows timezone to be dynamic (useful for traveling users).

Changes:
- Remove timezone field from User entity
- Remove timezone from user registration
- GET /habits/today now requires ?timezone= query param
- Add migration to drop timezone column from database
- Update related tests

internal/domain/entities/user.go

@@ -6,7 +6,6 @@ type User struct {
 	ID                      string
 	Email                   string
 	PasswordHash            string
-	Timezone                string
 	EmailVerified           bool
 	EmailVerificationToken  *string
 	EmailVerificationExpiry *time.Time
@@ -14,15 +13,11 @@ type User struct {
 	UpdatedAt               time.Time
 }
 
-func NewUser(email, passwordHash, timezone string) *User {
+func NewUser(email, passwordHash string) *User {
 	now := time.Now()
-	if timezone == "" {
-		timezone = "UTC"
-	}
 	return &User{
 		Email:        email,
 		PasswordHash: passwordHash,
-		Timezone:     timezone,
 		CreatedAt:    now,
 		UpdatedAt:    now,
 	}

internal/domain/entities/user_test.go

@@ -8,9 +8,8 @@ import (
 func TestNewUser(t *testing.T) {
 	email := "test@example.com"
 	passwordHash := "hashed_password_123"
-	timezone := "Europe/Madrid"
 
-	user := NewUser(email, passwordHash, timezone)
+	user := NewUser(email, passwordHash)
 
 	if user.Email != email {
 		t.Errorf("Expected email %s, got %s", email, user.Email)
@@ -20,10 +19,6 @@ func TestNewUser(t *testing.T) {
 		t.Errorf("Expected password hash %s, got %s", passwordHash, user.PasswordHash)
 	}
 
-	if user.Timezone != timezone {
-		t.Errorf("Expected timezone %s, got %s", timezone, user.Timezone)
-	}
-
 	if user.CreatedAt.IsZero() {
 		t.Error("CreatedAt should not be zero")
 	}
@@ -37,11 +32,3 @@ func TestNewUser(t *testing.T) {
 		t.Errorf("CreatedAt and UpdatedAt should be nearly identical, diff: %v", diff)
 	}
 }
-
-func TestUser_DefaultTimezone(t *testing.T) {
-	user := NewUser("test@example.com", "hash", "")
-
-	if user.Timezone != "UTC" {
-		t.Errorf("Expected default timezone UTC, got %s", user.Timezone)
-	}
-}