RecentDmConversationsPage: Design / implement appearance when empty

[chrisbobbe]

When the user doesn't have any recent DM conversations, RecentDmConversationsPage (titled "Direct messages") should have a message saying so, instead of just being blank:

Design

We have a design for this in Figma:
https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=5956-115842&t=dosxggnU7UlsrbfI-0

Screenshot:

For details, see the Figma doc.

The design relies on having a way to start a new DM conversation:

• Support starting a new DM conversation #127

So this issue won't be ready to work on until that one is done.

Related issues

• InboxPage: Design / implement appearance when empty #386
• Support starting a new DM conversation #127

[chrisbobbe]

Once we do this (or #386, for the inbox), let's revisit SubscriptionListPage (titled "Streams") to make sure it looks consistent with what we do here. Here's what that page currently does:

[sm-sayedi]

I would like to work on this issue!

So far that I have worked on it, it just shows a message like this at the center of the page:

[chrisbobbe]

Thanks for your interest, @sm-sayedi! I think this issue isn't ripe to work on yet, though. We'll want our designer to suggest how this should look.

[alya]

We should keep the text consistent with the web app, unless there's a good reason for them to be different:

You have no direct messages yet! Why not start the conversation?

The rest of the visual design can be as in Figma.

[chrisbobbe]

I've split #1551 off from this, for some polish in the Figma (graphics), and sent #1552 for a text-only placeholder, marking that as fixing this issue.