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/i18n/locale/en/conversation.json

@@ -309,7 +309,8 @@
       "JOIN_CALL": "Join call",
       "END_CALL": "End call",
       "MUTE": "Mute mic",
-      "UNMUTE": "Unmute mic"
+      "UNMUTE": "Unmute mic",
+      "VIEW_CHAT_HISTORY": "View chat history"
     }
   },
   "EMAIL_TRANSCRIPT": {