style(ftmap): one world, themed ground — here and on every map

The FT map wrapped the planet into a row of copies and showed Leaflet's
hard-coded grey beyond it. noWrap tiles inside hard bounds now, like the
grid-square map already did — and the container background override moves to
style.css so EVERY map's surround follows the theme's card surface instead
of #ddd (the grid map's per-div fix was losing to leaflet.css specificity).

frontend/src/style.css

@@ -1086,3 +1086,10 @@
 .overflow-scroll {
   overscroll-behavior: contain;
 }
+
+/* The ground beside the planet, on every map: Leaflet paints its container a
+   hard-coded light grey (#ddd), which reads as a broken tile against any
+   theme. The surround follows the theme's own card surface instead. */
+.leaflet-container {
+  background: var(--card) !important;
+}