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/ui/src/components/toggle.rs

@@ -1,6 +1,6 @@
 use gpui::{
-    AnyElement, AnyView, ClickEvent, CursorStyle, ElementId, Hsla, IntoElement, Styled, Window,
-    div, hsla, prelude::*,
+    AnyElement, AnyView, ClickEvent, ElementId, Hsla, IntoElement, Styled, Window, div, hsla,
+    prelude::*,
 };
 use std::sync::Arc;
 
@@ -610,7 +610,7 @@ impl RenderOnce for SwitchField {
         h_flex()
             .id(SharedString::from(format!("{}-container", self.id)))
             .when(!self.disabled, |this| {
-                this.hover(|this| this.cursor(CursorStyle::PointingHand))
+                this.hover(|this| this.cursor_pointer())
             })
             .w_full()
             .gap_4()