PM-5043: Restore locked challenge fee fallback

[jmgasper]

What was broken
Locked challenge rows in the billing account details modal could show a blank Challenge Fee value when challenge markup hydration did not resolve, even though the row still had the billing-account markup needed to calculate the fee.

Root cause
The prior PM-5043 modal fix moved challenge rows to challenge-specific markup, but canonical challenge rows returned undefined while hydration was missing or failed. Locked rows store member-payment subtotals directly, so the fee calculation had no fallback markup and rendered "-".

What was changed
The modal now falls back to billing-account markup when challenge-specific markup is unavailable, while preserving hydrated challenge markup values including zero. This keeps locked challenge fees populated without changing consumed-row subtotal handling.

Any added/updated tests
Added BillingAccountLineItemsModal regression coverage for a locked challenge row whose challenge markup cannot be loaded, asserting the member payments remain unchanged and the Challenge Fee is still calculated.

Validation

• yarn test:no-watch --runTestsByPath src/apps/work/src/lib/components/BillingAccountLineItemsModal/BillingAccountLineItemsModal.spec.tsx passed.
• yarn lint passed.
• yarn run build passed with existing warnings.
• yarn test:no-watch was run and failed in unrelated wallet-admin, PaymentFormModal, PaymentView, and ChallengeEditorForm suites on current dev.