Stale merchant details can leak across logout/login sessions

[StuartFerguson]

Problem

MyAccountPageViewModel now prefers ApplicationCache.GetMerchantDetails() over fetching fresh merchant details from the API.

Impact

Merchant details are not cleared on logout, so a user who logs out and then logs in as a different merchant can see the previous merchant's name/details in My Account until the cache is refreshed elsewhere.

Reproduction

1. Log in as merchant A.
2. Open My Account so merchant details are cached.
3. Log out.
4. Log in as merchant B.
5. Open My Account.

Expected

My Account should show merchant B's details.

Actual

Cached details from merchant A can be reused.

Suggested fix

Clear cached merchant details on logout, or scope the cached value to the active session/merchant and always refresh when the session changes.

[StuartFerguson]

Clarification: the merchant details cache should be overwritten on a subsequent successful login, so the main risk here is stale data being visible between logout and the next completed login flow. Clearing the cached merchant details on logout is still a sensible preventative measure to avoid any accidental reuse before the new session repopulates it.