Fix permission denied error when updating allowed_nodes

The update_allowed_nodes function was changing ownership of the temp file
before all writes were complete, causing 'Permission denied' errors when
appending the allowed_nodes section.

Root cause:
- mktemp creates file owned by script runner (root)
- chown changed ownership to pulse-sensor-proxy:pulse-sensor-proxy
- Subsequent append (>>) failed because root can't write to the file

Fix: Defer chown until after all writes complete and file is moved to
final location. Ownership is still correctly set on the final config file.

scripts/install-sensor-proxy.sh

@@ -120,13 +120,11 @@ update_allowed_nodes() {
             }
         ' "$config_file" > "$tmp_config"
 
-        # Preserve file permissions
+        # Preserve file permissions (but defer ownership change until after all writes)
         chmod --reference="$config_file" "$tmp_config" 2>/dev/null || chmod 0644 "$tmp_config"
-        chown --reference="$config_file" "$tmp_config" 2>/dev/null || true
     else
         touch "$tmp_config"
         chmod 0644 "$tmp_config"
-        chown pulse-sensor-proxy:pulse-sensor-proxy "$tmp_config" 2>/dev/null || true
     fi
 
     # Append new allowed_nodes section