Use channel folders in inbox view

[alya]

In zulip/zulip#35002 / PR zulip/zulip#35152, we implemented channel folders in the inbox view in the web app. We should do the same on mobile.

Specifically, we should add channel folder sections, sorted as in the web app inbox: direct messages (which becomes styled as a folder rather than a channel), pinned channels, all the folders alphabetically, channels (i.e., uncategorized channels). Folders should be shown only when they contain unread channels/topics to show.

Behavior and styling

We should ideally get a design from @terpimost, but here's what I'm imagining:

• Click should expand/collapse the folder.
• Show unread count summaries on collapsed folders (aligned with unreads for channels/topics).
• On expanded folders, let's start by just showing unread summaries on DMs (like on web, though on web you can also get unread count summaries on hover).
• Put folder names on the background color, and make the contents of each folder a card.
• Align the collapse/expand triangle with the ones for folders, and align the folder name with

CZO discussion: #mobile-team > channel folders on mobile