feat(#175): validate fiat code before submitting an order

lib/core/daemon_errors.dart

@@ -44,6 +44,12 @@ String localizedDaemonError(
   if (raw.contains('TradeNotDisputable')) {
     return l10n.tradeNotDisputable;
   }
+  // The fiat code failed the create-order preflight (#175): a stale or tampered
+  // saved default that is not a valid ISO 4217 code. Re-picking a currency fixes
+  // it before the request is ever published.
+  if (raw.contains('InvalidFiatCode')) {
+    return l10n.invalidFiatCode;
+  }
   // A dispute for this trade already exists, or one is still in flight: the
   // open is a duplicate either way, and retrying it changes nothing.
   if (raw.contains('DisputeAlreadyOpen')) {

lib/l10n/app_de.arb

@@ -1,6 +1,7 @@
 {
   "@@locale": "de",
   "@@last_modified": "2026-03-31",
+  "invalidFiatCode": "Diese Währung wird nicht unterstützt. Bitte wähle vor dem Erstellen der Order eine andere.",
 
   "appName": "Mostro",
   "loading": "Laden…",

lib/l10n/app_en.arb

@@ -1,6 +1,10 @@
 {
   "@@locale": "en",
   "@@last_modified": "2026-03-31",
+  "invalidFiatCode": "That currency isn't supported. Please pick another before creating the order.",
+  "@invalidFiatCode": {
+    "description": "Shown when a create-order fiat code fails the ISO 4217 preflight (stale/tampered saved default)"
+  },
 
   "appName": "Mostro",
   "@appName": {"description": "Application name"},

lib/l10n/app_es.arb

@@ -1,6 +1,7 @@
 {
   "@@locale": "es",
   "@@last_modified": "2026-03-31",
+  "invalidFiatCode": "Esa moneda no es compatible. Elige otra antes de crear la orden.",
 
   "appName": "Mostro",
   "loading": "Cargando…",

lib/l10n/app_fr.arb

@@ -1,6 +1,7 @@
 {
   "@@locale": "fr",
   "@@last_modified": "2026-03-31",
+  "invalidFiatCode": "Cette devise n'est pas prise en charge. Choisissez-en une autre avant de créer l'ordre.",
 
   "appName": "Mostro",
   "loading": "Chargement…",

lib/l10n/app_it.arb

@@ -1,6 +1,7 @@
 {
   "@@locale": "it",
   "@@last_modified": "2026-03-31",
+  "invalidFiatCode": "Quella valuta non è supportata. Scegline un'altra prima di creare l'ordine.",
 
   "appName": "Mostro",
   "loading": "Caricamento…",

rust/src/api/settings.rs

@@ -86,8 +86,14 @@ fn validate_locale(locale: &str) -> Result<()> {
     }
 }
 
-/// Validates an ISO 4217 fiat code: exactly 3 uppercase ASCII letters.
-fn validate_fiat_code(code: &str) -> Result<()> {
+/// Validates the *syntactic* shape of an ISO 4217 fiat code: exactly 3
+/// uppercase ASCII letters. This does not check membership in any supported
+/// currency set — a well-formed but unsupported code (e.g. "XYZ") passes here
+/// and is left for the daemon to reject. Membership validation against the
+/// daemon's advertised `supported_currencies` is tracked as a follow-up
+/// (#175 review) so there is a single authoritative source rather than a
+/// bundled list that can drift.
+pub(crate) fn validate_fiat_code(code: &str) -> Result<()> {
     let valid = code.len() == 3 && code.chars().all(|c| c.is_ascii_uppercase());
     if valid {
         Ok(())
@@ -352,6 +358,30 @@ mod tests {
         assert!(err.to_string().contains("InvalidFiatCode"));
     }
 
+    /// #175: create_order preflights the fiat code with this same validator, so
+    /// a stale or tampered saved default is rejected locally with the stable
+    /// InvalidFiatCode marker instead of going out as a daemon CantDo. Covers
+    /// the format cases that reach the create path.
+    #[test]
+    fn validate_fiat_code_marker_cases() {
+        // Valid ISO 4217 shape passes.
+        assert!(validate_fiat_code("USD").is_ok());
+        assert!(validate_fiat_code("EUR").is_ok());
+        // Bad shapes all fail with the InvalidFiatCode marker.
+        for bad in ["", "US", "USDD", "usd", "Us1", "US$", "ドル"] {
+            let err = validate_fiat_code(bad).unwrap_err();
+            assert!(
+                err.to_string().contains("InvalidFiatCode"),
+                "{bad:?} must be rejected with the InvalidFiatCode marker"
+            );
+        }
+        // A well-formed but unsupported code passes syntax validation — this
+        // documents the boundary so the guarantee is not misread as "rejects
+        // unsupported currencies". Membership is a tracked follow-up (#175
+        // review), enforced by the daemon in the meantime.
+        assert!(validate_fiat_code("XYZ").is_ok());
+    }
+
     #[tokio::test]
     async fn set_default_fiat_code_none_clears() {
         let _g = settings_lock().lock().unwrap();

test/core/daemon_errors_test.dart

@@ -75,6 +75,24 @@ void main() {
     );
   });
 
+  /// PR #304 review (Catrya): the InvalidFiatCode preflight marker must map to
+  /// its localized string like every other daemon-error marker, both bare and
+  /// with the offending code as context.
+  test('maps the InvalidFiatCode preflight marker (#175)', () {
+    expect(
+      localizedDaemonError(l10n, 'InvalidFiatCode', fallback: 'x'),
+      l10n.invalidFiatCode,
+    );
+    expect(
+      localizedDaemonError(
+        l10n,
+        "InvalidFiatCode: 'XYZ' must be exactly 3 uppercase ASCII letters (ISO 4217)",
+        fallback: 'x',
+      ),
+      l10n.invalidFiatCode,
+    );
+  });
+
   test('maps timeout and storage markers, and falls back otherwise', () {
     expect(
       localizedDaemonError(l10n, 'NoDaemonResponse', fallback: 'x'),