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/src/lib/api.ts

@@ -116,4 +116,20 @@ export const importConfig = callable<[], {
     hardwareSlug: string | null
   }
   error?: string
-}>("import_config")
+}>("import_config")
+
+// ── Plugin Pairing ──────────────────────────────────────────
+export const initiatePair = callable<[baseUrl?: string], {
+  success: boolean
+  token?: string
+  qrUrl?: string
+  expiresAt?: string
+  error?: string
+}>("initiate_pair")
+
+export const checkPairStatus = callable<[token: string, baseUrl?: string], {
+  status: "pending" | "confirmed" | "expired" | "invalid"
+  apiKey?: string
+  keyName?: string
+  error?: string
+}>("check_pair_status")