build: align pinned Node.js version with CI runtime (26.8.1)

.nvmrc and package.json engines.node must match the exact version the
CI environment runs, otherwise scripts/check-node-toolchain.mjs fails
the strict equality assertion.

.nvmrc

@@ -1 +1 @@
-26.7.0
+26.8.1

package.json

@@ -3,7 +3,7 @@
 	"private": true,
 	"type": "module",
 	"engines": {
-		"node": ">=26.7.0 <27"
+		"node": ">=26.8.1 <27"
 	},
 	"packageManager": "[email protected]",
 	"scripts": {