feat(sponsors): drop the sidebar slot and simplify the Sponsors page

The sidebar no longer carries a sponsor card. The backend stops serving
the "sidebar" slot, so a sponsors.json entry that still lists it keeps
only its other slots and one listing nothing else is dropped. Its
rotation, three-sponsor cap and collapsed-rail logo rendering go with it.

The Sponsors page now lists only real sponsors: the "Your brand here"
card shows only as the empty state instead of sitting in the grid and
repeating the header's Become a sponsor button, and the Available
placements section and its slot-occupancy helper are removed. Cards are
larger, two per row on wide screens. Unused i18n keys are dropped from
every locale.

internal/web/translation/es-ES.json

@@ -2423,19 +2423,7 @@
       "tag": "Patrocinador",
       "visit": "Visitar",
       "yourBrand": "Tu marca aquí",
-      "yourBrandText": "Muestra tu empresa a miles de administradores de servidores que abren 3X-UI cada día.",
-      "placements": "Espacios disponibles",
-      "placementDashboard": "Banner del panel",
-      "placementDashboardDesc": "Espacio premium en la parte superior del panel, un patrocinador a la vez.",
-      "placementSidebar": "Tarjeta lateral",
-      "placementSidebarDesc": "Se muestra en todas las páginas del panel, rotando entre patrocinadores.",
-      "placementLogin": "Página de inicio de sesión",
-      "placementLoginDesc": "La ve el administrador en cada inicio de sesión.",
-      "placementPage": "Página de patrocinadores",
-      "placementPageDesc": "Aparece aquí con logo, descripción y enlace.",
-      "available": "Disponible",
-      "takenUntil": "Ocupado hasta {date}",
-      "activeCount": "{count} activos"
+      "yourBrandText": "Muestra tu empresa a miles de administradores de servidores que abren 3X-UI cada día."
     }
   },
   "tgbot": {