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/widgets/FloatingCallWidget.vue

@@ -77,15 +77,12 @@ const handleJoinCall = async call => {
   if (!call || isJoining.value) return;
   const { conversation } = getCallInfo(call);
 
-  // End current active call before joining new one
   if (hasActiveCall.value) {
     await handleEndCall();
   }
 
-  // After a hard refresh the conversation may not be hydrated yet — but the call
-  // object already carries inboxId from the cable / seeding path, so accept can
-  // proceed without it. Twilio still needs inboxId for initializeDevice; falls
-  // back to the conversation's inbox_id when present.
+  // The conversation may not be hydrated yet (post-refresh seeding path);
+  // call.inboxId already carries what joinCall needs.
   const result = await joinCall({
     conversationId: call.conversationId,
     inboxId: call.inboxId || conversation?.inbox_id,