refactor(live-atc): extract normalizeManualFreq to shared/utils, add tests

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

shared/utils/frequency.ts

@@ -0,0 +1,10 @@
+// Accepts inputs like "121.5", "121,500" or "118" and normalises to a valid
+// VHF airband frequency string (118.000–136.975). Returns null when invalid so
+// callers/UI can disable the action.
+export function normalizeManualFreq(input: string): string | null {
+  const raw = input.trim().replace(',', '.')
+  if (!raw) return null
+  const num = Number(raw)
+  if (!Number.isFinite(num) || num < 118 || num >= 137) return null
+  return num.toFixed(3)
+}