refactor(live-atc): extract ATIS playback/loop scheduling to useAtisPlayback

Moves ATIS carrier/broadcast loop, TTS audio caching, METAR fallback,
and the background airport-data refresh scheduler out of live-atc.vue.
Also extracts the pmLog debug logger to shared/utils/pmLog.ts since it
has no Vue/component dependency and multiple composables need it —
threading it through every composable's parameter list would've been
worse than giving it one shared home. Verified via typecheck plus a
browser mount/redirect smoke test.

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

shared/utils/pmLog.ts

@@ -0,0 +1,26 @@
+// ---------------------------------------------------------------------------
+// Debug logger — prefix [PM] so it's easy to filter in DevTools.
+// Disable at runtime: localStorage.setItem('PM_DEBUG', '0')
+// Re-enable:          localStorage.setItem('PM_DEBUG', '1')
+// Show trace detail:  localStorage.setItem('PM_DEBUG', 'verbose')
+// ---------------------------------------------------------------------------
+export const pmLog = (() => {
+  const level = () => {
+    if (typeof localStorage === 'undefined') return 'on'
+    return localStorage.getItem('PM_DEBUG') ?? 'on'
+  }
+  const enabled  = () => level() !== '0'
+  const verbose  = () => level() === 'verbose'
+  return {
+    info:  (...a: any[]) => enabled() && console.log   ('[PM]', ...a),
+    warn:  (...a: any[]) => enabled() && console.warn  ('[PM]', ...a),
+    error: (...a: any[]) => enabled() && console.error ('[PM]', ...a),
+    debug: (...a: any[]) => verbose() && console.debug ('[PM:v]', ...a),
+    group: (label: string, fn: () => void) => {
+      if (!enabled()) { fn(); return }
+      console.groupCollapsed(`[PM] ${label}`)
+      fn()
+      console.groupEnd()
+    },
+  }
+})()