feat(machine): production safety — disable mock fallback and ndebit

When VITE_ALLOW_MOCK_FALLBACK is unset (production default), the ATM
now shows a maintenance screen instead of silently falling back to mock
services when hardware or Lightning initialization fails. Also disables
ndebit/CLINK in production since the static ndebit pointer is replayable
— cash-in uses LNURL-withdraw only (single-use by design).

- Add allowMockFallback config field (Electron IPC + types)
- Add strict config validation (no localhost, require private key)
- Gate all catch-block fallbacks behind allowMockFallback
- Disable debit approval service and ndebit generation in production
- Add maintenance screen in App.vue when initError is set

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

apps/machine/electron/preload.ts

@@ -24,6 +24,7 @@ export interface RuntimeConfig {
   validatorDevice?: string
   dispenserDevice?: string
   cassettes?: string
+  allowMockFallback: boolean
 }
 
 // Expose protected methods to renderer