[Leo] - Change placeholder text and distinguish new and ongoing conversations

[aguscruiz]

Placeholder copy for new conversations input box should change to something more inviting: "What do you want to accomplish today?".
After the first message is sent, the placeholder changes to "Ask follow-up".

This should affect:

• Sidebar
• Full page
• Android
• iOS

New conversation:

Ongoing conversation:

Thread:
https://bravesoftware.slack.com/archives/C04KPE0BUDU/p1731345117609659