feat(machine): add renderer watchdog for kiosk resilience

After 6 days of uptime the Electron renderer silently crashed while the
main process kept running (blank screen, no recovery). Three-layer
detection: render-process-gone (instant), unresponsive (Chromium), and
IPC heartbeat (30s ping, 2 missed = reload). Reloads renderer via
loadFile/loadURL preserving HAL hardware state in main process.

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

apps/machine/src/App.vue

@@ -26,6 +26,13 @@ const formattedBtcPrice = computed(() => {
 })
 
 onMounted(async () => {
+  // Watchdog: respond to heartbeat pings from main process
+  if (window.electronAPI?.onWatchdogPing) {
+    window.electronAPI.onWatchdogPing(() => {
+      window.electronAPI!.watchdogPong()
+    })
+  }
+
   try {
     if (isElectron) {
       await atmStore.initializeForProduction()

apps/machine/src/types/electron.d.ts

@@ -60,6 +60,8 @@ declare global {
       onHalBillInserted: (callback: (denomination: number) => void) => void
       onHalBillRejected: (callback: (reason: string) => void) => void
       onHalError: (callback: (error: string) => void) => void
+      onWatchdogPing: (callback: () => void) => void
+      watchdogPong: () => Promise<void>
       platform: NodeJS.Platform
     }
   }