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.

The issue names no files, tests, or entry points. Start by locating the existing customer account, cart, order, and routing code, then trace how identity and sessions currently work. Done means the listed acceptance criteria pass, including account isolation, secure logout, simultaneous use, and prevention of unauthorized access.

Written by the indexing model from the issue text.