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/store/modules/contacts/actions.js

@@ -312,10 +312,14 @@ export const actions = {
     commit(types.CLEAR_CONTACT_FILTERS);
   },
 
-  initiateCall: async ({ commit }, { contactId, inboxId }) => {
+  initiateCall: async ({ commit }, { contactId, inboxId, conversationId }) => {
     commit(types.SET_CONTACT_UI_FLAG, { isInitiatingCall: true });
     try {
-      const response = await ContactAPI.initiateCall(contactId, inboxId);
+      const response = await ContactAPI.initiateCall(
+        contactId,
+        inboxId,
+        conversationId
+      );
       commit(types.SET_CONTACT_UI_FLAG, { isInitiatingCall: false });
       return response.data;
     } catch (error) {