nginx: serve mgmt /static/ assets from disk (no Flask round-trip)

Generated management-domain server blocks now include a
location /static/ aliasing webui/static/ with Cache-Control: no-cache
(ETag revalidation -> 304), nosniff, and a restrictive CSP, so SPA
asset requests no longer reach Flask. Flask's static route remains
the dev-mode fallback.

- lib/nginx.py + daemon/handlers/nginx.py: static_root render context
  (the handler renders the template directly, so both paths need it)
- template: alias uses a trailing slash (nginx concatenates the
  location remainder onto the alias value)
- install.sh: a+rX on webui/static plus execute-up-the-parent-chain
  so the nginx worker (www-data) can traverse repo-in-$HOME installs
- tests: mgmt static location assertions (positive + non-mgmt negative)
- docs: AGENTS.md, architecture.md, security.md static-serving notes

lib/nginx.py

@@ -416,6 +416,7 @@ def generate_server_conf(
         cert_key_path=cert_key_path,
         domain_auth=domain_auth,
         has_management=has_management,
+        static_root=str(PROJECT_DIR / "webui" / "static"),
         acme_cert_dir=acme_cert_dir,
         certs_dir=str(PROJECT_DIR / "data" / "certs"),
         acme_webroot=str(PROJECT_DIR / "data" / "acme" / "www"),