fix: stabilize pay point of sale flow

[k9ert]

Split from #157 to keep the Pay POS route/test stabilization separate from the admin phone-rate-limit reset work.

Context

The POS flow had a route replacement loop risk because query params were compared by object identity. The Cypress test also depended on fixture account state that was not established inside the test.

Changes

• Compare current POS query params by value before replacing the route.
• Create and verify the POS Cypress username/default-wallet fixture before exercising the cash register flow.

Validation

• buck2 test //apps/pay:lint
• buck2 build //apps/pay:build-ci
• Pay app CI passed on the split PR.
• The failed SDL job was rerun because the log showed an external Rover plugin download timeout, not a code failure.
• BATS passed after rerunning an unrelated onchain-send broadcast retry failure.