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/components/sponsor/SponsorCard.css

@@ -184,28 +184,37 @@
 .sponsor-card-card .sponsor-main {
   flex-direction: column;
   align-items: flex-start;
-  padding: 16px;
+  gap: 16px;
+  padding: 24px;
 }
 
 .sponsor-card-card .sponsor-logo {
-  width: 56px;
-  height: 56px;
+  width: 72px;
+  height: 72px;
+  border-radius: 12px;
+}
+
+.sponsor-card-card .sponsor-logo-fallback {
+  font-size: 28px;
+}
+
+.sponsor-card-card .sponsor-body {
+  gap: 6px;
+}
+
+.sponsor-card-card .sponsor-title {
+  font-size: 17px;
+  line-height: 1.4;
+}
+
+.sponsor-card-card .sponsor-text {
+  font-size: 14px;
+  line-height: 1.6;
 }
 
 .sponsor-card-card .sponsor-visit {
   margin-top: auto;
-}
-
-.sponsor-card-icon {
-  justify-content: center;
-  padding: 6px;
-  border-color: transparent;
-  background: transparent;
-}
-
-.sponsor-card-icon .sponsor-logo {
-  width: 32px;
-  height: 32px;
+  font-size: 14px;
 }
 
 @media (max-width: 576px) {