feat(machine): persist scanned spire-seed + signal unpaired state for wizard

Foundation for the on-machine QR-pairing wizard (aiolabs/bitspire#52). An
unpaired ATM can now have a seed planted at runtime rather than only via
provisioning:

- electron IPC `state:save-spire-seed` writes VITE_SPIRE_SEED into the runtime
  .env (0600), and `app:relaunch` restarts the kiosk so the normal boot path
  (signer-resolver → connectNewSeed) does the actual bunker pairing. We
  deliberately do NOT pair in-renderer — persist + relaunch reuses the single,
  hardware-tested pairing path.
- signer-resolver throws a typed `NoPairingError` (distinct `.name`, survives
  the bundle boundary) when there's no seed and no binding, instead of a
  generic Error.
- init-error maps NoPairingError → `unpaired`, so the renderer can route a
  fresh machine to the interactive wizard (next commit) rather than a
  dead-end fault screen. Revoked/TTL bindings already map there too — re-pair
  is the same scan-a-fresh-seed flow.

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

apps/machine/electron/main.ts

@@ -353,6 +353,50 @@ ipcMain.handle('state:reset-bootstrap-gate', (): void => {
   resetBootstrapGate()
 })
 
+// QR-pairing wizard (aiolabs/bitspire#52): an unpaired machine scans a
+// spire-seed off its camera, and we persist it as VITE_SPIRE_SEED in the
+// runtime .env so the next boot's signer-resolver redeems it (connectNewSeed)
+// exactly as if it had been provisioned. We deliberately do NOT pair here —
+// persisting + relaunching reuses the single, tested pairing path rather than
+// duplicating it in the renderer.
+function runtimeEnvPath(): string {
+  const base = fs.existsSync('/var/lib/bitspire') ? '/var/lib/bitspire' : process.cwd()
+  return path.join(base, '.env')
+}
+
+ipcMain.handle('state:save-spire-seed', (_event, seed: string): void => {
+  const trimmed = (seed || '').trim()
+  if (!trimmed) throw new Error('save-spire-seed: empty seed')
+  const envPath = runtimeEnvPath()
+  const line = `VITE_SPIRE_SEED=${trimmed}`
+  let lines: string[] = []
+  if (fs.existsSync(envPath)) {
+    lines = fs.readFileSync(envPath, 'utf8').split('\n')
+  }
+  const idx = lines.findIndex((l) => l.startsWith('VITE_SPIRE_SEED='))
+  if (idx >= 0) {
+    lines[idx] = line
+  } else {
+    // Drop a trailing empty element so we don't accumulate blank lines.
+    if (lines.length && lines[lines.length - 1] === '') lines.pop()
+    lines.push(line)
+  }
+  fs.writeFileSync(envPath, lines.join('\n') + '\n', { mode: 0o600 })
+  // Keep this process's view in sync so get-atm-secrets reflects the new seed
+  // even before relaunch (belt-and-suspenders; relaunch re-reads from disk).
+  process.env.VITE_SPIRE_SEED = trimmed
+  console.log('[Pairing] Spire seed persisted to', envPath)
+})
+
+// Relaunch the kiosk so the new seed is picked up by a clean boot. Under
+// systemd (bitspire.service) the exit triggers an automatic restart; in dev
+// Electron's relaunch re-spawns the process.
+ipcMain.handle('app:relaunch', (): void => {
+  console.log('[Pairing] Relaunching to apply new pairing')
+  app.relaunch()
+  app.exit(0)
+})
+
 // State persistence IPC handlers
 ipcMain.handle('state:load-cassettes', () => loadCassettes())
 ipcMain.handle('state:set-cassettes', (_event, cassettes) => setCassettes(cassettes))