Refactor nginx to path-based domain model with config migration

Replace the legacy top-level management key with a unified paths-based
model. Each domain now contains a paths map where each entry defines its
own backend, auth, headers, and flags (is_management, is_websocket).

- Add _migrate_config() to auto-migrate legacy formats on first load
- Remove set_management_proxy() and POST_NGINX_MANAGEMENT endpoint
- Update server_block.conf template to iterate paths with per-location auth
- Update daemon handler, API blueprint, state collector, and install script
- Add server config generation tests for paths, WebSocket, auth inheritance
- Update frontend proxy page to display per-path rows with flags

webui/server.py

@@ -197,6 +197,7 @@ def spa_root():
 def vendor_files(filename):
     """Serve vendored JS libraries (htm.js, etc.)."""
     from flask import send_file
+
     target = (VENDOR_DIR / filename).resolve()
     if not target.is_relative_to(VENDOR_DIR):
         abort(404)