fix(exchange): stop one unparseable order from wiping the order list - #2531

⚠️ Requires emulator/device review before merge — draft until verified on a real account. Test: log into an account whose history contains an Expired-status order (the reporter's account is the fixture) and confirm the transactions list shows all exchange transactions (Pay, Reward, etc.), rewards show real amounts instead of 0 sats.

Problem

The API legitimately sends order values the app refused to parse: status Expired (distinct from Payment deadline expired), payout status Failed, order type Sell USDT, empty payin/payout statuses, and null exchange-rate/deadline fields on admin-initiated orders. Both batch parse layers (datasource listOrderSummaries and repository getOrders) turned the first such order into an empty list, hiding every exchange transaction in the app. Confirmed in device logs: Error fetching orders — Exception: Unknown OrderStatus: Expired. Server contract verified against API-Orders orderSummary.ts.

Changes

• Parse per element with skip-and-log at both layers, and in getOrderByTxId
• Unknown-tolerant fromValue on OrderStatus/OrderPayinStatus/OrderPayoutStatus/OrderType; add missing members (Expired, Failed, Sell USDT)
• Unknown order types render generically via a new Order.generic variant carrying the server-sent type name
• exchangeRateAmount/exchangeRateCurrency/confirmationDeadline now nullable (matching the server), with guards at consumers
• Reward amounts read from the payout side (payin is empty for admin-initiated orders → rendered 0 sats)
• Fiat-vs-sats display derived from the order instead of a hardcoded variant list (fixes fiat refunds shown as sats, BTC balance adjustments formatted as fiat)

Validation

15 new regression tests (poisoned-list survival at both layers, each new enum value, null fields, reward amount); flutter analyze clean; reviewed with a verification pass (2 should-fixes applied). Note: this branch touches Countdown null-guards in buy/pay/sell screens — minor line-disjoint overlap with the sibling fix PRs at merge.

Closes #2526
Closes #2527