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')

apps/web/drizzle/meta/_journal.json

@@ -197,6 +197,13 @@
       "when": 1782593630361,
       "tag": "0027_slimy_stryfe",
       "breakpoints": true
+    },
+    {
+      "idx": 28,
+      "version": "7",
+      "when": 1782655457218,
+      "tag": "0028_thin_bloodstrike",
+      "breakpoints": true
     }
   ]
 }