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.

frontend/src/layouts/AppSidebar.css

@@ -247,10 +247,6 @@
   padding: 8px 8px 12px;
 }
 
-.sider-sponsor {
-  margin-bottom: 6px;
-}
-
 .sidebar-pin {
   display: inline-flex;
   align-items: center;

frontend/src/layouts/AppSidebar.tsx

@@ -44,7 +44,6 @@ import { formatPanelVersion } from '@/lib/panel-version';
 import { pauseAnimationsUntilLeave, useTheme } from '@/hooks/useTheme';
 import { useAllSettings } from '@/api/queries/useAllSettings';
 import { useCommandPalette } from '@/components/command-palette/useCommandPalette';
-import SponsorSlot from '@/components/sponsor/SponsorSlot';
 import './AppSidebar.css';
 
 const DONATE_URL = 'https://donate.sanaei.dev/';
@@ -452,13 +451,6 @@ export default function AppSidebar() {
           onClick={onMenuClick}
         />
         <div className="sider-footer">
-          <SponsorSlot
-            slot="sidebar"
-            variant="compact"
-            iconOnly={railCollapsed}
-            rotate
-            className="sider-sponsor"
-          />
           <VersionBadge version={panelVersion} collapsed={railCollapsed} />
         </div>
       </Layout.Sider>
@@ -544,7 +536,6 @@ export default function AppSidebar() {
           }}
         />
         <div className="drawer-footer">
-          <SponsorSlot slot="sidebar" variant="compact" rotate className="sider-sponsor" />
           <VersionBadge version={panelVersion} />
         </div>
       </Drawer>