fix(dashboard): use subprocess JSON output for v5 toggle read-back
The daemon's toggle endpoint was reading config.json from a hardcoded legacy path that doesn't exist on plugin installs, causing the Structure Map checkbox to snap back to OFF. Replaced inline config read with --json subprocess output using _get_v5_feature_status() as single source of truth. Also fixed dashboard JS to fall back to optimistic toggle when the response has an empty v5_features map.
Closes #34