fix(kpa): a settings save must not flick the KPA500's power switch — and the maps stay under the menus

startAmps tore down and rebuilt every amplifier client on any settings
save, and closing a COM port makes the Windows driver drop DTR/RTS. On a
KPA500 those lines ARE the power switch — so 'save' meant 'amplifier off
twenty seconds later', exactly F1TPL's report, and the fix that held the
lines while OPEN couldn't survive a close. An amplifier whose config is
unchanged now keeps its running client across saves; the others still
rebuild.

Also: isolate on the FT Map panel — Leaflet's z-1000 panes were painting
over the menu bar and the Preferences dialog, as the grid map's own
comment warned.

frontend/src/components/FTMapPanel.tsx

@@ -119,7 +119,10 @@ export function FTMapPanel({ decodes, myGrid }: { decodes: FTMapDecode[]; myGrid
 
   const bands = [...new Set(decodes.map((d) => (d.band ?? '').toLowerCase()).filter(Boolean))];
   return (
-    <div className="relative h-full w-full min-h-0">
+    // isolate: Leaflet stacks its panes and controls up to z-index 1000, which
+    // beat the app menus and the Settings dialog. A stacking context of our own
+    // keeps all of it inside this panel.
+    <div className="relative isolate z-0 h-full w-full min-h-0">
       <div ref={divRef} className="absolute inset-0 rounded-lg overflow-hidden" />
       {/* Basemap picker, MainMap's own vocabulary. */}
       <div className="absolute top-2 left-12 z-[1000] flex gap-1 rounded-md bg-background/85 backdrop-blur px-1 py-1 border border-border">