onboarding ui: Add theme preview tiles and button functionality to basic page (#35413)

This PR polishes and adds functionality to the onboarding UI with a
focus on the basic page. It added theme preview tiles, got the Vim,
telemetry, crash reporting, and sign-in button working.

The theme preview component was moved to the UI crate and it now can
have a click handler on it.

Finally, this commit also changed `client::User.github_login` and
`client::UserStore.by_github_login` to use `SharedStrings` instead of
`Strings`. This change was made because user.github_login was cloned in
several areas including the UI, and was cast to a shared string in some
cases too.

Release Notes:

- N/A

---------

Co-authored-by: Remco Smits <djsmits12@gmail.com>

crates/channel/src/channel_store.rs

@@ -126,7 +126,7 @@ impl ChannelMembership {
                 proto::channel_member::Kind::Member => 0,
                 proto::channel_member::Kind::Invitee => 1,
             },
-            username_order: self.user.github_login.as_str(),
+            username_order: &self.user.github_login,
         }
     }
 }