feat(machine): pairing-source abstraction + QR/NFC capture + seed ingest

The capture half of the QR-pairing wizard (aiolabs/bitspire#52), behind a
`PairingSource` seam so the wizard UI stays agnostic to how the seed arrives:

- `QrPairingSource` — camera capture + decode via `qr` (paulmillr). Chosen
  over the dormant, unmaintained `jsqr`: `qr` is zero-dependency, auditable,
  dual MIT/Apache, actively maintained, and authored by the same person as the
  `@noble`/`@scure` crypto our nostr stack already trusts. Its `qr/dom.js`
  helper wraps getUserMedia + the per-frame decode loop.
- `NfcPairingSource` — Web NFC scaffold; `isAvailable()` is false on the
  Sintra's Linux Electron, so it's inert until real NFC hardware lands (the
  user flagged NFC as a plausible future pairing method).
- `ingestScannedSeed` — validates the scan parses as a spire-seed (rejecting a
  stray QR), persists it, and relaunches. Covered by unit tests
  (invalid-seed / no-bridge / persist-failed / happy path).
- `availablePairingSources()` probes each source and returns the runnable ones
  in preference order (camera first).

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

apps/machine/package.json

@@ -37,6 +37,7 @@
     "marked": "^17.0.5",
     "nostr-tools": "^2.10.0",
     "pinia": "^2.2.0",
+    "qr": "^0.6.0",
     "qrcode.vue": "^3.6.0",
     "reka-ui": "^2.7.0",
     "tailwind-merge": "^3.4.0",