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/0028_thin_bloodstrike.sql

@@ -0,0 +1,12 @@
+CREATE TABLE "plugin_pairings" (
+	"token" text PRIMARY KEY NOT NULL,
+	"user_id" text,
+	"api_key_id" text,
+	"api_key" text,
+	"status" text DEFAULT 'pending' NOT NULL,
+	"created_at" timestamp DEFAULT now() NOT NULL,
+	"confirmed_at" timestamp,
+	"expires_at" timestamp NOT NULL
+);
+--> statement-breakpoint
+CREATE INDEX "plugin_pairings_userId_idx" ON "plugin_pairings" USING btree ("user_id");