feat: QR code phone pairing for Decky plugin + MangoHud guide revision + SteamOS version fix

- Add plugin_pairings table + migration (0028) for short-lived pairing sessions
- Add pairing API: POST /api/plugin/pair/initiate, GET /status/:token, POST /confirm
- Add /pair web page: user scans QR on phone, confirms, API key auto-created
- Plugin: 'Pair with Phone' button renders QR code (qrcode.react), polls for status, saves API key
- Plugin: new initiate_pair + check_pair_status Python RPCs
- Revise MangoHud Setup Guide into clean numbered steps
- Fix SteamOS version detection: read VERSION_ID + BUILD_ID (was just 'SteamOS')

plugins/decky-vault/package.json

@@ -10,6 +10,7 @@
   "dependencies": {
     "@decky/api": "^1.1.3",
     "@deckyvault/shared": "workspace:*",
+    "qrcode.react": "^4.2.0",
     "react-icons": "^5.3.0",
     "tslib": "^2.7.0"
   },