feat: Ctrl+Up/Down hops spot-to-spot on the Main-view band map

Add a keyNav prop to BandMap: when set (only the docked Main-view map, not the
multi-band Band Map tab), Ctrl+ArrowUp / Ctrl+ArrowDown selects the next in-band
spot above / below the rig frequency and tunes to it via the existing spot-click
handler. Higher freq is up on the map (freqToY), so Up = next higher spot.
Ignored while typing in an input. Changelog 0.21.1.

frontend/src/App.tsx

@@ -5320,6 +5320,7 @@ export default function App() {
               currentFreqHz={band && freqMhz ? Math.round(parseFloat(freqMhz) * 1_000_000) : 0}
               onSpotClick={handleSpotClick}
               onClose={() => setBandMapShown(false)}
+              keyNav
             />
           </div>
         )}