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/zh-TW.json

@@ -2423,19 +2423,7 @@
       "tag": "贊助",
       "visit": "造訪",
       "yourBrand": "您的品牌展示位",
-      "yourBrandText": "讓每天打開 3X-UI 的數千名伺服器管理員看到您的公司。",
-      "placements": "可用展示位",
-      "placementDashboard": "總覽橫幅",
-      "placementDashboardDesc": "儀表板頂部的高級位置，每次僅一家贊助商。",
-      "placementSidebar": "側邊欄卡片",
-      "placementSidebarDesc": "在面板所有頁面顯示，並在贊助商之間輪換。",
-      "placementLogin": "登入頁",
-      "placementLoginDesc": "管理員每次登入時都能看到。",
-      "placementPage": "贊助商頁面",
-      "placementPageDesc": "在此頁面展示標誌、簡介與連結。",
-      "available": "可預訂",
-      "takenUntil": "已預訂至 {date}",
-      "activeCount": "{count} 個進行中"
+      "yourBrandText": "讓每天打開 3X-UI 的數千名伺服器管理員看到您的公司。"
     }
   },
   "tgbot": {