fix(ftmap): tell Leaflet when its box changes size; fix(decodes): all seven continents

The FT map drew a strip of dead space along the bottom after a zoom.
Leaflet measures its container ONCE, at creation — which for this panel
is the instant the tab is selected, before the flex layout has settled —
and then asks for tiles to fit that stale size for ever. A ResizeObserver
hands it the real size, plus one pass after the first paint for the
layout that settles without ever firing a resize.

And the decode list's continent chips are the seven, fixed. Building
them from the feed meant a row that reshuffled itself every period — a
chip appeared when the first Asian station decoded and moved everything
sideways under the pointer — and it could not say what the filter was
capable of until something had been heard.

frontend/src/components/FTMapPanel.tsx

@@ -62,7 +62,25 @@ export function FTMapPanel({ decodes, myGrid }: { decodes: FTMapDecode[]; myGrid
     });
     mapRef.current = m;
     layerRef.current = L.layerGroup().addTo(m);
-    return () => { m.remove(); mapRef.current = null; layerRef.current = null; };
+    // Leaflet measures its container ONCE, when the map is created, and then
+    // draws tiles for that size for ever. This panel is mounted the moment its
+    // tab is selected — before the flex layout has settled — and the window can
+    // be resized under it, so the stale measurement showed as a strip of dead
+    // space along the bottom where tiles were never asked for. The observer
+    // hands it the real size whenever the box changes.
+    const ro = new ResizeObserver(() => m.invalidateSize({ animate: false }));
+    ro.observe(divRef.current);
+    // Once more after the first paint: the first observation can arrive while
+    // the panel is still zero-height, and no further resize follows a layout
+    // that settles by itself.
+    const settle = window.setTimeout(() => m.invalidateSize({ animate: false }), 100);
+    return () => {
+      window.clearTimeout(settle);
+      ro.disconnect();
+      m.remove();
+      mapRef.current = null;
+      layerRef.current = null;
+    };
   }, []);
 
   // Basemap follows the picker.