Fix Ed25519 signature: use standard mode, not prehashed

EdDSAEngine(MessageDigest) is for Ed25519ph (prehashed mode).
Standard Ed25519 requires EdDSAEngine() with no arguments.

app/src/main/java/com/inou/clawdnode/security/DeviceIdentity.kt

@@ -101,8 +101,8 @@ class DeviceIdentity(context: Context) {
         val privateKeySpec = EdDSAPrivateKeySpec(privateKeyBytes, ed25519Spec)
         val privateKey = EdDSAPrivateKey(privateKeySpec)
         
-        // Sign the payload
-        val signature = EdDSAEngine(MessageDigest.getInstance(ed25519Spec.hashAlgorithm)).apply {
+        // Sign the payload using standard Ed25519 (not prehashed Ed25519ph)
+        val signature = EdDSAEngine().apply {
             initSign(privateKey)
             update(payload.toByteArray(Charsets.UTF_8))
         }