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/commands/register_user_test.go

@@ -74,7 +74,6 @@ func TestRegisterUserHandler_Success(t *testing.T) {
 	cmd := RegisterUserCommand{
 		Email:    "test@example.com",
 		Password: "Secure123!",
-		Timezone: "UTC",
 	}
 
 	result, err := handler.Handle(context.Background(), cmd)
@@ -97,10 +96,6 @@ func TestRegisterUserHandler_Success(t *testing.T) {
 	if createdUser.Email != cmd.Email {
 		t.Errorf("expected email %q, got %q", cmd.Email, createdUser.Email)
 	}
-
-	if createdUser.Timezone != cmd.Timezone {
-		t.Errorf("expected timezone %q, got %q", cmd.Timezone, createdUser.Timezone)
-	}
 }
 
 func TestRegisterUserHandler_InvalidEmail(t *testing.T) {
@@ -125,7 +120,6 @@ func TestRegisterUserHandler_InvalidEmail(t *testing.T) {
 			cmd := RegisterUserCommand{
 				Email:    tt.email,
 				Password: "Secure123!",
-				Timezone: "UTC",
 			}
 
 			_, err := handler.Handle(context.Background(), cmd)
@@ -160,38 +154,6 @@ func TestRegisterUserHandler_InvalidPassword(t *testing.T) {
 			cmd := RegisterUserCommand{
 				Email:    "test@example.com",
 				Password: tt.password,
-				Timezone: "UTC",
-			}
-
-			_, err := handler.Handle(context.Background(), cmd)
-			if !errors.Is(err, appErrors.ErrInvalidInput) {
-				t.Errorf("expected ErrInvalidInput, got %v", err)
-			}
-		})
-	}
-}
-
-func TestRegisterUserHandler_InvalidTimezone(t *testing.T) {
-	repo := &mockUserRepo{}
-	hasher := &mockPasswordHasher{}
-	handler := NewRegisterUserHandler(repo, hasher, nil, "", "open", false)
-
-	tests := []struct {
-		name     string
-		timezone string
-	}{
-		{"empty timezone", ""},
-		{"invalid timezone", "InvalidTimezone"},
-		{"numeric format", "GMT+1"},
-		{"partial timezone", "America"},
-	}
-
-	for _, tt := range tests {
-		t.Run(tt.name, func(t *testing.T) {
-			cmd := RegisterUserCommand{
-				Email:    "test@example.com",
-				Password: "Secure123!",
-				Timezone: tt.timezone,
 			}
 
 			_, err := handler.Handle(context.Background(), cmd)
@@ -203,7 +165,7 @@ func TestRegisterUserHandler_InvalidTimezone(t *testing.T) {
 }
 
 func TestRegisterUserHandler_EmailAlreadyExists(t *testing.T) {
-	existingUser := entities.NewUser("test@example.com", "hashed", "UTC")
+	existingUser := entities.NewUser("test@example.com", "hashed")
 	repo := &mockUserRepo{
 		findByEmailFunc: func(ctx context.Context, email string) (*entities.User, error) {
 			return existingUser, nil
@@ -215,7 +177,6 @@ func TestRegisterUserHandler_EmailAlreadyExists(t *testing.T) {
 	cmd := RegisterUserCommand{
 		Email:    "test@example.com",
 		Password: "Secure123!",
-		Timezone: "UTC",
 	}
 
 	_, err := handler.Handle(context.Background(), cmd)
@@ -237,7 +198,6 @@ func TestRegisterUserHandler_PasswordHashingError(t *testing.T) {
 	cmd := RegisterUserCommand{
 		Email:    "test@example.com",
 		Password: "Secure123!",
-		Timezone: "UTC",
 	}
 
 	_, err := handler.Handle(context.Background(), cmd)
@@ -259,7 +219,6 @@ func TestRegisterUserHandler_RepositoryError(t *testing.T) {
 	cmd := RegisterUserCommand{
 		Email:    "test@example.com",
 		Password: "Secure123!",
-		Timezone: "UTC",
 	}
 
 	_, err := handler.Handle(context.Background(), cmd)
@@ -283,7 +242,6 @@ func TestRegisterUserHandler_EdgeCases(t *testing.T) {
 			RegisterUserCommand{
 				Email:    "user+tag@example.com",
 				Password: "Secure123!",
-				Timezone: "UTC",
 			},
 			nil,
 		},
@@ -292,16 +250,6 @@ func TestRegisterUserHandler_EdgeCases(t *testing.T) {
 			RegisterUserCommand{
 				Email:    "user@mail.example.com",
 				Password: "Secure123!",
-				Timezone: "UTC",
-			},
-			nil,
-		},
-		{
-			"complex timezone",
-			RegisterUserCommand{
-				Email:    "user@example.com",
-				Password: "Secure123!",
-				Timezone: "America/Argentina/Buenos_Aires",
 			},
 			nil,
 		},
@@ -310,7 +258,6 @@ func TestRegisterUserHandler_EdgeCases(t *testing.T) {
 			RegisterUserCommand{
 				Email:    "user@example.com",
 				Password: "Sëcure123!",
-				Timezone: "UTC",
 			},
 			nil,
 		},
@@ -319,7 +266,6 @@ func TestRegisterUserHandler_EdgeCases(t *testing.T) {
 			RegisterUserCommand{
 				Email:    "user@example.com",
 				Password: "ValidP@ss1" + string(make([]byte, 100)),
-				Timezone: "UTC",
 			},
 			nil,
 		},
@@ -342,7 +288,6 @@ func TestRegisterUserHandler_ClosedRegistration(t *testing.T) {
 	cmd := RegisterUserCommand{
 		Email:    "test@example.com",
 		Password: "Secure123!",
-		Timezone: "UTC",
 	}
 
 	_, err := handler.Handle(context.Background(), cmd)