feat(voice): unify call button flow and redesign call widget

- Contact-panel call button now continues in the open conversation when
  the conversation's inbox is voice-capable, mirroring the header button.
  For non-voice channels it falls back to the inbox picker.
- Twilio and WhatsApp paths share the same upstream decision; both pass
  a conversationId hint to the provider so the agent stays in the same
  thread.
- Floating call widget redesigned: avatar + name + phone with duration
  on top, a clickable "View chat history" pill linking to the
  conversation, and labeled End / Join action buttons. Mute is preserved
  for active WhatsApp calls.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/javascript/dashboard/api/contacts.js

@@ -47,9 +47,10 @@ class ContactAPI extends ApiClient {
     return axios.get(`${this.url}/${contactId}/labels`);
   }
 
-  initiateCall(contactId, inboxId) {
+  initiateCall(contactId, inboxId, conversationId = null) {
     return axios.post(`${this.url}/${contactId}/call`, {
       inbox_id: inboxId,
+      conversation_id: conversationId,
     });
   }