home: Add leading/trailing graphics to empty-page placeholders

[chrisbobbe]

For launch, we'll simplify the empty-page placeholders for these pages:

• Inbox: InboxPage: Design / implement appearance when empty #386
• Channels
• Direct messages: RecentDmConversationsPage: Design / implement appearance when empty #385

by just displaying some plain text. That's much better than just an empty space.

These have designs in Figma, though, with nice-looking SVGs. This issue is for incorporating those graphics.

Inbox	Channels	Direct messages

Figma links:

Inbox: https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=5966-176662&m=dev
Channels: https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=6205-25948&m=dev
Direct messages: https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=5956-115842&m=dev

Related

• inbox: Change empty-inbox text so it links to "Combined feed" and "Channels" #1553