Use icon for topic-list button in app bar

[gnprice]

Our initial implementation of #1158, in #1500, uses the word "TOPICS" for the app-bar action/button leading to the topic-list page, following the design in Figma.

We want to change it to use a "list" icon instead. This is in order to let it occupy less space, especially in languages where the translation of "TOPICS" might be long.

References

• Chat thread: #mobile > Flutter beta: missing topic list @ 💬

[chrisbobbe]

Do we know which specific icon we want?

Current "TOPICS" button:

Could be replaced with, e.g.:

https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=8996-27909&m=dev

(are those two identical?)

Or e.g.:

cc @terpimost @alya

[chrisbobbe]

zulip/zulip#34890 is a Help Center PR that's written for the app before this change; we should update it after this change lands :)