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')

bun.lock

@@ -83,10 +83,11 @@
     },
     "plugins/decky-vault": {
       "name": "@deckyvault/plugin",
-      "version": "0.1.0",
+      "version": "1.0.0",
       "dependencies": {
         "@decky/api": "^1.1.3",
         "@deckyvault/shared": "workspace:*",
+        "qrcode.react": "^4.2.0",
         "react-icons": "^5.3.0",
         "tslib": "^2.7.0",
       },
@@ -1669,6 +1670,8 @@
 
     "pvutils": ["pvutils@1.1.5", "", {}, "sha512-KTqnxsgGiQ6ZAzZCVlJH5eOjSnvlyEgx1m8bkRJfOhmGRqfo5KLvmAlACQkrjEtOQ4B7wF9TdSLIs9O90MX9xA=="],
 
+    "qrcode.react": ["qrcode.react@4.2.0", "", { "peerDependencies": { "react": "^16.8.0 || ^17.0.0 || ^18.0.0 || ^19.0.0" } }, "sha512-QpgqWi8rD9DsS9EP3z7BT+5lY5SFhsqGjpgW5DY/i3mK4M9DTBNz3ErMi8BWYEfI3L0d8GIbGmcdFAS1uIRGjA=="],
+
     "queue-microtask": ["queue-microtask@1.2.3", "", {}, "sha512-NuaNSa6flKT5JaSYQzJok04JzTL1CA6aGhv5rfLW3PgqA+M2ChpZQnAC8h8i4ZFkBS8X5RqkDBHA7r4hej3K9A=="],
 
     "react": ["react@19.2.6", "", {}, "sha512-sfWGGfavi0xr8Pg0sVsyHMAOziVYKgPLNrS7ig+ivMNb3wbCBw3KxtflsGBAwD3gYQlE/AEZsTLgToRrSCjb0Q=="],