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

Design in Figma: https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=13188-51396&t=1rJA5zq21eG7riyW-0

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

[chrisbobbe]

Updated the issue description to point to the new Figma design.