Customer Authentication & Session Management

## Description

Implement a secure authentication and session management system for customers using the public-facing Resto POS website.

Each customer must have an independent account and session so that multiple customers can use the system simultaneously without accessing each other's personal information.

## Requirements

* Implement Google Authentication for customer accounts.
* Create or associate each Google account with a unique customer ID.
* Use server-side sessions to identify the currently authenticated customer.
* Ensure customers can only access their own:

* Profile
* Cart
* Orders
* Personal information
* Prevent customers from accessing another customer's data by modifying URLs, requests, or customer IDs.
* Support multiple customers using the system at the same time.
* Provide secure login and logout functionality.

## Example

```text
Customer A → Google Account → Customer ID 101 → Session A
Customer B → Google Account → Customer ID 102 → Session B
```

Both customers may access the same pages, but each session must display only the data belonging to its respective customer.

## Acceptance Criteria

* [ ] Google Authentication works for customers.
* [ ] Each customer has a unique account/ID.
* [ ] Customer sessions are handled securely.
* [ ] Customers can only access their own data.
* [ ] Multiple customers can use the system simultaneously.
* [ ] Logout properly terminates the session.
* [ ] Unauthorized access to another customer's data is prevented.

No files, tests, or entry points are named in the issue. Start by surveying the PHP/MySQL authentication, session, and customer-data entry points. Done means the acceptance criteria are verified, including isolated customer data, secure login and logout, simultaneous sessions, and blocked unauthorized access.

索引モデルが issue の本文から書いたものです。