Add customer system integration (login, profile, details)

[jorgenosberg]

The example app currently has no concept of an authenticated customer. To demonstrate a realistic mobility app experience, we need to integrate a customer system across the BFF and Flutter client — login, viewing profile details, and updating customer information.

This involves a separate /customers API (not part of the OMSA specification) that handles:

• Authentication (login/logout flow)
• Fetching customer profile (name, email, phone, traveler preferences)
• Updating customer details
• Linking customer identity to OMSA operations (e.g. associating purchases with a customer, pre-filling traveler info)

Since this repo is a public demo, the /customers API should be mocked (similar to #48) — no real identity provider or customer backend required. The mock should return realistic Wayfare-branded customer data and support basic CRUD operations in-memory.

BFF work:

• New routes for proxying/handling customer endpoints
• Session or token management for the mock auth flow
• Connecting customer context to OMSA calls where relevant (e.g. traveler profiles from customer data)

Flutter work:

• Login screen / auth flow
• Profile screen updates to show real customer data instead of placeholder content
• Edit profile functionality
• Use customer data to pre-fill traveler information in search