Update UI: Implement inbox design for empty state

[yashrajpu]

This pull request (PR) addresses issue #386 and enhances the user interface by introducing a conditional feature for displaying messages.

If messages are available, they are displayed in the UI. If no messages exist, a default text is shown instead. This improvement enhances the user experience by providing clear feedback when no messages are present.

Changes Made:

• Implemented logic to check for the existence of messages.
• Displayed messages when available; if not, switched to a default text.
• Improved the overall layout and clarity of the UI.

Testing:

• Verified that the feature works correctly when messages are present.
• Confirmed the fallback text appears appropriately when no messages are available.

Figma link:-

The updated UI aligns with the Figma design provided here

[Figma Design Link]
(https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=5966-176642&p=f&t=gdUmGEmNiTNiCHVf-0)

Ensured consistent behaviour across various screen sizes and edge cases.

Screenshots:

Old image:

New image:

[apoorvapendse]

https://chat.zulip.org/#narrow/channel/44-git-help/topic/pull.20request/near/2050861

[apoorvapendse]

Closing #1293 as promised.

[chrisbobbe]

Thanks for PR. Before we can review in detail, this needs:

• Tests
• Follow the project style for code indentation
• If rebasing atop autocomplete test: Add extra pump in new test for new RawAutocomplete delay #1290, keep the commit from autocomplete test: Add extra pump in new test for new RawAutocomplete delay #1290 without squashing unrelated changes into it
• Remove from the branch any unrelated changes (so, keep the commit for autocomplete test: Add extra pump in new test for new RawAutocomplete delay #1290 if it's not in main, but remove the ndkVersion change and anything else that's not relevant)

If you need help with Git, #git help in the development community is the best place to ask.

[chrisbobbe]

In your commit and PR description (two places), please also say Fixes #1294..

[yashrajpu]

Could you please check this for me? Thank you!

[alya]

Is the font in the second line bigger? They look the same to me in Figma.

[alya]

Let's make it "inbox" rather than "Inbox", for consistency with Zulip's capitalization style.

[alya]

Please also add light-theme screenshot to the PR description.

[alya]

I'm not sure we've got a good call to action in the second line; I'll post on the CZO thread.