fix(editor): a corrected frequency corrects its band

Band and frequency have to agree — the log, every award and every upload
are read on the BAND — and an operator fixing a wrong frequency is not
also expecting to fix the band by hand. That is exactly how a QSO ends
up filed on 20m at 7 MHz. Both sides follow, TX and RX.

Only when the number lands in a known allocation, because half a
frequency is typed on the way to all of it and a band must never be
blanked by that. bandForMHz moves out of App.tsx into lib/bandplan,
where the two callers can share one answer instead of drifting.

frontend/src/lib/bandplan.ts

@@ -111,3 +111,26 @@ export function bandRange(band: string): [number, number] | undefined {
 export function bandSegments(band: string): Seg[] {
   return plan().segments[band] ?? [];
 }
+
+// bandForMHz maps a dial frequency (MHz) to its ADIF band, or '' when it falls
+// outside every known allocation.
+//
+// Lives here rather than in a panel because more than one place has to answer
+// the same question the same way: the entry form retunes on a typed frequency,
+// and the QSO editor has to keep band and frequency agreeing when one of them
+// is corrected. Kept in step with BandFromHz on the Go side.
+export function bandForMHz(mhz: number): string {
+  if (!mhz || isNaN(mhz)) return '';
+  const plan: [number, number, string][] = [
+    [1.8, 2.0, '160m'], [3.5, 4.0, '80m'], [5.06, 5.45, '60m'], [7.0, 7.3, '40m'],
+    [10.1, 10.15, '30m'], [14.0, 14.35, '20m'], [18.068, 18.168, '17m'], [21.0, 21.45, '15m'],
+    [24.89, 24.99, '12m'], [28.0, 29.7, '10m'], [50, 54, '6m'], [70, 71, '4m'],
+    [144, 148, '2m'], [222, 225, '1.25m'], [420, 450, '70cm'], [902, 928, '33cm'], [1240, 1300, '23cm'],
+    // Microwave, ADIF 3.1.7 ranges.
+    [2300, 2450, '13cm'], [3300, 3500, '9cm'], [5650, 5925, '6cm'], [10000, 10500, '3cm'],
+    [24000, 24250, '1.25cm'], [47000, 47200, '6mm'], [75500, 81000, '4mm'],
+    [119980, 123000, '2.5mm'], [134000, 149000, '2mm'], [241000, 250000, '1mm'],
+  ];
+  for (const [lo, hi, b] of plan) if (mhz >= lo && mhz <= hi) return b;
+  return '';
+}