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/ja-JP.json

@@ -2423,19 +2423,7 @@
       "tag": "スポンサー",
       "visit": "訪問",
       "yourBrand": "ここにあなたのブランドを",
-      "yourBrandText": "毎日 3X-UI を開く何千人ものサーバー管理者に御社をアピールできます。",
-      "placements": "掲載枠",
-      "placementDashboard": "ダッシュボードバナー",
-      "placementDashboardDesc": "ダッシュボード上部のプレミアム枠。一度に 1 社のみ。",
-      "placementSidebar": "サイドバーカード",
-      "placementSidebarDesc": "パネルの全ページに表示され、スポンサー間でローテーションします。",
-      "placementLogin": "ログインページ",
-      "placementLoginDesc": "管理者がログインするたびに表示されます。",
-      "placementPage": "スポンサーページ",
-      "placementPageDesc": "ロゴ、説明、リンク付きでここに掲載されます。",
-      "available": "空きあり",
-      "takenUntil": "{date} まで予約済み",
-      "activeCount": "{count} 件掲載中"
+      "yourBrandText": "毎日 3X-UI を開く何千人ものサーバー管理者に御社をアピールできます。"
     }
   },
   "tgbot": {