Validate fiat code before submitting market-price orders

[ermeme]

Milestone: M6 - UX / minor parity
Type: bug / parity
Priority: low

App v2 currently takes the selected fiat code from the order form and sends it directly into NewOrderParams.fiat_code without a submit-time validation step.

The currency picker is backed by the bundled fiat list, which is good for UX, but it is still only a UI constraint. If the saved default fiat code becomes stale or is tampered with, the create-order request can still go out with an unsupported code and come back as a daemon-side CantDo instead of being rejected locally.

Relevant code

• lib/features/order/screens/add_order_screen.dart:157-180 - reads selectedFiatCodeProvider and builds NewOrderParams.
• lib/features/order/widgets/currency_section.dart:53-67 - picker selection path.
• rust/src/api/types.rs:217-233 - NewOrderParams has fiat_code, but no preflight validation hook.

Expected behavior

• Reject unsupported fiat codes before the request is published.
• Keep the UX aligned with the actual create-order contract instead of relying only on the picker list.

[grunch]

Code verification (2026-07-16)

Valid: add_order_screen.dart (157-180) reads selectedFiatCodeProvider straight into NewOrderParams with no submit-time validation, and NewOrderParams (types.rs:217-233) has no validation hook. Low is right — the picker constrains the happy path; this is defense-in-depth against a stale/tampered saved default. Suggest validating in Rust (new_order rejects codes not in the bundled fiat list with a stable marker, e.g. InvalidFiatCode, that Dart localizes) so every future caller inherits the check — same pattern as the BondRequired marker.