Stable v2.6.3

• fix(aliases): pre-fill www. on domain select to prevent full-domain retyping
• fix(ssl,domains): show actual resolved IP + bidirectional alias cleanup
• docs(plan): mark Phase 2/3 hardening + upstream repos done
• feat(agent): nginx.org upstream repo (opt-in) + own the vhost includes
• fix(sites): sites table fits narrower widths (progressive responsive columns)
• fix(changelog): keep full pre-stable history in the public changelog
• fix(docs): restore stable-version.json — panel fetches it to filter the stable changelog
• feat(changelog): separate stable and testing changelog lines