chore(frontend): DO NOT MERGE — add allowance mode to signer failure simulator

Adds the 'allowance' mode (exhausted per-user ICRC-2 allowance) so the new
sign.error.limit_reached message can be reproduced alongside payment/internal/
signing. Merged latest PR #13145 so the limit_reached path is present.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: sbpublic

src/frontend/src/lib/utils/signer-failure-simulator.utils.ts

@@ -1,7 +1,9 @@
 import { browser } from '$app/environment';
+import type { PaymentError } from '$declarations/signer/signer.did';
 import { CanisterInternalError } from '$lib/canisters/errors';
 import { SignerCanisterPaymentError } from '$lib/canisters/signer.errors';
 import { LOCAL, STAGING } from '$lib/constants/app.constants';
+import { Principal } from '@dfinity/principal';
 import { isNullish } from '@dfinity/utils';
 
 /**
@@ -19,11 +21,16 @@ import { isNullish } from '@dfinity/utils';
  *   - or localStorage:  `localStorage.setItem('OISY_SIMULATE_SIGNER_FAILURE', 'payment')`
  *
  * The query param wins over localStorage. Set the value to `off` (or clear it) to disable.
+ *
+ * Modes: `payment` (backend out of cycles → "signer unavailable"), `allowance` (exhausted
+ * per-user allowance → "signing limit reached"), `internal` / `signing` (generic non-payment
+ * signer errors → fallback toast).
  */
-export type SimulatedSignerFailureMode = 'payment' | 'internal' | 'signing';
+export type SimulatedSignerFailureMode = 'payment' | 'allowance' | 'internal' | 'signing';
 
 const SIMULATED_SIGNER_FAILURE_MODES: SimulatedSignerFailureMode[] = [
 	'payment',
+	'allowance',
 	'internal',
 	'signing'
 ];
@@ -63,6 +70,16 @@ export const simulateSignerFailureIfEnabled = () => {
 		});
 	}
 
+	if (mode === 'allowance') {
+		// Exhausted per-user ICRC-2 allowance → the "signing limit reached" toast.
+		throw new SignerCanisterPaymentError({
+			LedgerWithdrawFromError: {
+				error: { InsufficientAllowance: { allowance: 1_000n } },
+				ledger: Principal.anonymous()
+			}
+		} as unknown as PaymentError);
+	}
+
 	if (mode === 'signing') {
 		// A non-payment signer error (e.g. a threshold-signing failure) → fallback toast.
 		throw new CanisterInternalError('Simulated signer SigningError (demo)');