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/en-US.json

@@ -2423,19 +2423,7 @@
       "tag": "Sponsor",
       "visit": "Visit",
       "yourBrand": "Your brand here",
-      "yourBrandText": "Put your company in front of thousands of server admins who open 3X-UI every day.",
-      "placements": "Available placements",
-      "placementDashboard": "Overview banner",
-      "placementDashboardDesc": "Premium spot at the top of the dashboard, one sponsor at a time.",
-      "placementSidebar": "Sidebar card",
-      "placementSidebarDesc": "Shown on every panel page, rotating between sponsors.",
-      "placementLogin": "Login page",
-      "placementLoginDesc": "Seen by the admin at every sign-in.",
-      "placementPage": "Sponsors page",
-      "placementPageDesc": "Listed here with logo, description and link.",
-      "available": "Available",
-      "takenUntil": "Taken until {date}",
-      "activeCount": "{count} active"
+      "yourBrandText": "Put your company in front of thousands of server admins who open 3X-UI every day."
     }
   },
   "tgbot": {