fix: restore full admin panel design with clean URLs
## Changes
- Restored original admin.html with full design (3251 lines)
- Updated redirect URLs from /login.html to /login
- Maintained all sidebar sections (Principal, Contenido, Analítica, Sistema)
- Kept all 33 sidebar navigation links
## Admin Sections
- Principal: Dashboard, Overview, Properties, Favorites
- Contenido: Properties, Leads, Testimonials, FAQ, Services, Blog
- Analítica: Analytics, Reports
- Sistema: Users, Settings, Logs
## URLs
- /login → login.html
- /admin → admin.html
- Redirect after auth to /admin (not /admin.html)
## Verified
✅ Full admin design restored
✅ All menu sections present
✅ Clean URLs working
✅ Auth redirects correctly
0708853251