chore(voice): trim dead exports, fix duplicate-OR bug, drop redundant comments

Branch review pass:
- useWhatsappCallSession: drop unused `error` ref and unused exports
  (hasActiveWhatsappCall, isWhatsappCallMuted), fold sendWhatsappCallBeacon
  into a private beaconTerminate helper since only sendWhatsappTerminateBeacon
  consumed it externally. Trim WHAT-comments; keep WHY-comments
  (browser-quirk explanations, race-condition notes, auth-cookie rationale).
- VoiceCall.vue bubble: fix `data || data` short-circuit that did nothing —
  upstream key transform was the same on both branches; collapse to one read.
- calls/useCallSession/actionCable/FloatingCallWidget/VoiceCallButton:
  drop comments that just describe what the next line already says.

Net diff: -63 lines across 7 files. No behavior change.

app/javascript/dashboard/components-next/Contacts/VoiceCallButton.vue

@@ -44,7 +44,6 @@ const voiceInboxes = computed(() =>
 );
 const hasVoiceInboxes = computed(() => voiceInboxes.value.length > 0);
 
-// Unified behavior: hide when no phone
 const shouldRender = computed(() => hasVoiceInboxes.value && !!props.phone);
 
 const isInitiatingCall = computed(() => {
@@ -128,7 +127,6 @@ const startCall = async inboxId => {
     });
     const { call_sid: callSid, conversation_id: conversationId } = response;
 
-    // Add call to store immediately so widget shows
     const callsStore = useCallsStore();
     callsStore.addCall({
       callSid,