gpui: Add rounded_md token (#26179)

This PR adds a new rounded/corner border token: `rounded_md` with a
value of 6px.

I feel like I was wanting to use 6px border radius a lot but avoiding
due to it being an arbitrary value... so, not anymore! It's also cool to
have this be consistent with Tailwind v4.

Follow on to the prior renames:

- `rounded_sm` -> `rounded_xs`:
https://github.com/zed-industries/zed/pull/26221
- `rounded_md` -> `rounded_sm`:
https://github.com/zed-industries/zed/pull/26228

Release Notes:

- N/A

---------

Co-authored-by: Marshall Bowers <git@maxdeviant.com>

crates/assistant2/src/active_thread.rs

@@ -674,7 +674,7 @@ impl ActiveThread {
                         .pr_2()
                         .bg(cx.theme().colors().editor_foreground.opacity(0.02))
                         .when(is_open, |element| element.border_b_1().rounded_t(px(6.)))
-                        .when(!is_open, |element| element.rounded(px(6.)))
+                        .when(!is_open, |element| element.rounded_md())
                         .border_color(cx.theme().colors().border)
                         .child(
                             h_flex()