Offer a list of all channels, and a way to subscribe

[gnprice]

Sometimes one wants to navigate to a channel/stream when not already subscribed, or to subscribe to a channel.

Like in zulip-mobile, we'll offer this functionality as an "All channels" screen, reached from a button at the bottom of the "Channels" tab.

Design

This screen has a design in Figma:
https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=7723-6411&m=dev

See there for details, including some notes.

Differences from Figma:

• The design there shows a "New channel" button. That is a useful feature but will be a separate follow-up issue. (edit: Support creating a new channel #1572)
• The design there shows a "Filter channels" search box at the top. That is a useful feature but will be a separate follow-up issue.
• The design there shows a modified channel action sheet.
  • The action sheet, and the "..." button leading to it, are required as part of this issue. (The action sheet provides the way to navigate from this screen to a channel.)
  • The extra details at the top of the action sheet — channel icon, name, description, and statistics — will be a separate issue.

[opxdelwin]

Hey @gnprice I'd like to work on this issue. I'd love some help regarding the UI implementation.

• Should we show both subscribed and unsubscribed streams?

[gnprice]

Yes indeed — this screen should show all streams. (Well, all streams that the user is able to see.)

Cross-referencing, we have a more detailed discussion in this chat thread:
https://chat.zulip.org/#narrow/stream/48-mobile/topic/all-streams.20screen.20.23F188/near/1761243