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/id-ID.json

@@ -2423,19 +2423,7 @@
       "tag": "Sponsor",
       "visit": "Kunjungi",
       "yourBrand": "Merek Anda di sini",
-      "yourBrandText": "Tampilkan perusahaan Anda kepada ribuan admin server yang membuka 3X-UI setiap hari.",
-      "placements": "Slot yang tersedia",
-      "placementDashboard": "Banner dasbor",
-      "placementDashboardDesc": "Tempat premium di atas dasbor, satu sponsor dalam satu waktu.",
-      "placementSidebar": "Kartu sidebar",
-      "placementSidebarDesc": "Tampil di setiap halaman panel, bergantian antar sponsor.",
-      "placementLogin": "Halaman login",
-      "placementLoginDesc": "Dilihat admin setiap kali masuk.",
-      "placementPage": "Halaman sponsor",
-      "placementPageDesc": "Tercantum di sini dengan logo, deskripsi, dan tautan.",
-      "available": "Tersedia",
-      "takenUntil": "Terisi hingga {date}",
-      "activeCount": "{count} aktif"
+      "yourBrandText": "Tampilkan perusahaan Anda kepada ribuan admin server yang membuka 3X-UI setiap hari."
     }
   },
   "tgbot": {