fix: Use correct device auth payload format for signature

The signature payload was incorrect. Changed from:
  $nonce:$signedAt

To gateway's expected v2 format:
  v2|deviceId|clientId|clientMode|role|scopes|signedAtMs|token|nonce

This matches the buildDeviceAuthPayload() function in Clawdbot's
gateway/device-auth.js module.

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

@@ -20,7 +20,7 @@ import java.security.SecureRandom
  * The key format matches the Clawdbot gateway protocol:
  * - Public key: 32 bytes raw, base64url-encoded
  * - Signature: 64 bytes raw, base64url-encoded
- * - Payload: "$nonce:$signedAt" (matching gateway's expected format)
+ * - Payload format (v2): "v2|deviceId|clientId|clientMode|role|scopes|signedAtMs|token|nonce"
  */
 class DeviceIdentity(context: Context) {
     
@@ -68,13 +68,43 @@ class DeviceIdentity(context: Context) {
     }
     
     /**
-     * Sign a challenge nonce for gateway authentication
+     * Sign a challenge nonce for gateway authentication.
+     * 
+     * Builds the payload in gateway's expected format:
+     * v2|deviceId|clientId|clientMode|role|scopes|signedAtMs|token|nonce
+     * 
+     * @param nonce Challenge nonce from gateway
+     * @param clientId Client identifier (e.g., "clawdnode-android")
+     * @param clientMode Client mode ("node")
+     * @param role Role ("node")
+     * @param scopes Comma-separated scopes (empty for nodes)
+     * @param token Auth token (gateway token or device token)
      */
-    fun signChallenge(nonce: String): SignedChallenge {
+    fun signChallenge(
+        nonce: String,
+        clientId: String,
+        clientMode: String,
+        role: String,
+        scopes: String = "",
+        token: String = ""
+    ): SignedChallenge {
         ensureKeyPair()
         
         val signedAt = System.currentTimeMillis()
-        val payload = "$nonce:$signedAt"
+        
+        // Build payload in gateway's expected format:
+        // v2|deviceId|clientId|clientMode|role|scopes|signedAtMs|token|nonce
+        val payload = listOf(
+            "v2",
+            deviceId,
+            clientId,
+            clientMode,
+            role,
+            scopes,
+            signedAt.toString(),
+            token,
+            nonce
+        ).joinToString("|")
         
         Log.d(tag, "Signing payload: $payload")
         

app/src/main/java/com/inou/clawdnode/service/GatewayClient.kt

@@ -226,7 +226,16 @@ class GatewayClient(
         
         try {
             deviceId = deviceIdentity.deviceId
-            signedChallenge = deviceIdentity.signChallenge(nonce)
+            // Sign with full device auth payload:
+            // v2|deviceId|clientId|clientMode|role|scopes|signedAtMs|token|nonce
+            signedChallenge = deviceIdentity.signChallenge(
+                nonce = nonce,
+                clientId = Protocol.CLIENT_ID,
+                clientMode = Protocol.MODE,
+                role = Protocol.ROLE,
+                scopes = "",  // Nodes don't use scopes
+                token = token
+            )
             publicKey = deviceIdentity.publicKey
             log("Device identity ready: id=${deviceId.take(8)}...")
             log("DEBUG: ${signedChallenge.debugInfo}")