fix(machine): production hardware fixes for Douro cash-out

- Fix cassette denominations: Douro uses Q100/Q200, not Q20
- Add hal:get-inventory IPC so renderer can read HAL cassette inventory
- Add balance fetch/display to HAL+IPC init path and idle screen
- Enable/disable bill validator via watch on nested state transitions
- Pass fiatCode to state machine context (was hardcoded to USD)
- Preserve currency across state machine resetContext
- Add CANCEL handler to dispenseError state (was stuck)
- Fix remaining hardcoded $ symbols in CashInView

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

apps/machine/electron/preload.ts

@@ -57,6 +57,7 @@ contextBridge.exposeInMainWorld('electronAPI', {
   halDisableValidator: (): Promise<void> => ipcRenderer.invoke('hal:disable-validator'),
   halStackBill: (): Promise<void> => ipcRenderer.invoke('hal:stack-bill'),
   halRejectBill: (): Promise<void> => ipcRenderer.invoke('hal:reject-bill'),
+  halGetInventory: (): Promise<Record<number, number>> => ipcRenderer.invoke('hal:get-inventory'),
   halCleanup: (): Promise<void> => ipcRenderer.invoke('hal:cleanup'),
 
   // HAL event listeners (main process → renderer)
@@ -106,6 +107,7 @@ declare global {
       halDisableValidator: () => Promise<void>
       halStackBill: () => Promise<void>
       halRejectBill: () => Promise<void>
+      halGetInventory: () => Promise<Record<number, number>>
       halCleanup: () => Promise<void>
       onHalBillRead: (callback: (denomination: number) => void) => void
       onHalBillInserted: (callback: (denomination: number) => void) => void