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/application/queries/login_user.go

@@ -14,9 +14,8 @@ type LoginUserQuery struct {
 }
 
 type LoginUserResult struct {
-	UserID   string
-	Email    string
-	Timezone string
+	UserID string
+	Email  string
 }
 
 type LoginUserHandler struct {
@@ -50,8 +49,7 @@ func (h *LoginUserHandler) Handle(ctx context.Context, query LoginUserQuery) (*L
 	}
 
 	return &LoginUserResult{
-		UserID:   user.ID,
-		Email:    user.Email,
-		Timezone: user.Timezone,
+		UserID: user.ID,
+		Email:  user.Email,
 	}, nil
 }

internal/application/queries/login_user_test.go

@@ -55,7 +55,7 @@ func (m *mockLoginPasswordHasher) Compare(hashedPassword, password string) error
 }
 
 func TestLoginUserHandler_Success(t *testing.T) {
-	user := entities.NewUser("test@example.com", "hashed_password", "UTC")
+	user := entities.NewUser("test@example.com", "hashed_password")
 	user.ID = "user-123"
 	user.EmailVerified = true
 
@@ -150,7 +150,7 @@ func TestLoginUserHandler_UserNotFound(t *testing.T) {
 }
 
 func TestLoginUserHandler_InvalidPassword(t *testing.T) {
-	user := entities.NewUser("test@example.com", "hashed_password", "UTC")
+	user := entities.NewUser("test@example.com", "hashed_password")
 	user.ID = "user-123"
 	user.EmailVerified = true
 
@@ -180,7 +180,7 @@ func TestLoginUserHandler_InvalidPassword(t *testing.T) {
 }
 
 func TestLoginUserHandler_EmailNotVerified(t *testing.T) {
-	user := entities.NewUser("test@example.com", "hashed_password", "UTC")
+	user := entities.NewUser("test@example.com", "hashed_password")
 	user.ID = "user-123"
 	user.EmailVerified = false
 

internal/application/queries/refresh_token.go

@@ -17,9 +17,8 @@ type RefreshTokenQuery struct {
 }
 
 type RefreshTokenResult struct {
-	UserID   string
-	Email    string
-	Timezone string
+	UserID string
+	Email  string
 }
 
 type RefreshTokenHandler struct {
@@ -57,9 +56,8 @@ func (h *RefreshTokenHandler) Handle(ctx context.Context, query RefreshTokenQuer
 	}
 
 	return &RefreshTokenResult{
-		UserID:   user.ID,
-		Email:    user.Email,
-		Timezone: user.Timezone,
+		UserID: user.ID,
+		Email:  user.Email,
 	}, nil
 }
 

internal/application/queries/refresh_token_test.go

@@ -80,7 +80,7 @@ func TestRefreshTokenHandler_Success(t *testing.T) {
 
 	userRepo := &mockUserRepositoryForRefresh{
 		findByIDFunc: func(ctx context.Context, id string) (*entities.User, error) {
-			user := entities.NewUser("test@example.com", "hash", "UTC")
+			user := entities.NewUser("test@example.com", "hash")
 			user.ID = id
 			return user, nil
 		},