agent: Improve error and warnings display (#36425)

This PR refactors the callout component and improves how we display
errors and warnings in the agent panel, along with improvements for
specific cases (e.g., you have `zed.dev` as your LLM provider and is
signed out).

Still a work in progress, though, wrapping up some details.

Release Notes:

- N/A

crates/ai_onboarding/src/young_account_banner.rs

@@ -17,6 +17,6 @@ impl RenderOnce for YoungAccountBanner {
         div()
             .max_w_full()
             .my_1()
-            .child(Banner::new().severity(ui::Severity::Warning).child(label))
+            .child(Banner::new().severity(Severity::Warning).child(label))
     }
 }