fix(cash-in): prevent cancel after bills inserted in insertingBills state

The cancel button was still accessible during insertingBills after a bill
had been stacked (physically irreversible). Now CANCEL in insertingBills
is guarded: no bills → idle, bills present → confirmAbandon warning.
The UI also hides the cancel button once bills are detected.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/state-machine/src/machine.ts

@@ -426,8 +426,26 @@ export function createATMMachine(
                 guard: 'hasInsertedBills',
                 target: 'generatingNdebit',
               },
-              CANCEL: '#atm.idle',
-              TIMEOUT: '#atm.idle',
+              CANCEL: [
+                {
+                  // No bills inserted yet: safe to cancel
+                  guard: ({ context }) => context.billsInserted.length === 0,
+                  target: '#atm.idle',
+                },
+                {
+                  // Bills already stacked: warn user before abandoning
+                  target: 'confirmAbandon',
+                },
+              ],
+              TIMEOUT: [
+                {
+                  guard: ({ context }) => context.billsInserted.length === 0,
+                  target: '#atm.idle',
+                },
+                {
+                  target: 'confirmAbandon',
+                },
+              ],
             },
           },
           generatingNdebit: {