Redesign for autocomplete result lists

[gnprice]

From @terpimost we have a new design for how the autocomplete result lists should be presented:

Details are in Figma:
https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=3954-13395&m=dev

The design within each item in the results is a separate related issue:

• Redesign for @-mention autocomplete items #913

This issue is about how the result list as a whole is presented:

• the fading effect at the top and/or bottom when scrolled
• the list's height, including not going off the top of the screen (I think we may already successfully avoid this)
• its rounded corners, border, and shadow
• its placement (dipping vertically a few pixels into the top of the text field, and aligning horizontally with the width of messages in the message list)
• any other details I'm missing right now

This should apply to autocomplete both for the content input, and for the topic input.

[maiHydrogen]

@gnprice
hello there!!
i would really like to take the both issues to work upon if no one is assigned or working on it currently i'm kinda newbie in open source but i'm working with flutter for more than a year now.

[chrisbobbe]

Hi, welcome. This issue has the M6 Post-launch milestone, which means the core team isn't going to be able to spend time in the next couple of months to review or help with work on it. If you'd like to contribute to Zulip, see the guide in our README and the resources linked from there: https://github.com/zulip/zulip-flutter#picking-an-issue-to-work-on