(core) Add AI assistant usage banners

Summary:
Banners are now shown when there are low or no AI assistant
credits remaining.

Test Plan: Browser tests.

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D4018

app/client/components/Banner.ts

@@ -43,14 +43,14 @@ export interface BannerOptions {
   /**
    * If provided, applies the css class to the banner container.
    */
-   bannerCssClass?: string;
+  bannerCssClass?: string;
 
   /**
    * Function that is called when the banner close button is clicked.
    *
    * Should be used to handle disposal of the Banner.
    */
-   onClose?(): void;
+  onClose?(): void;
 }
 
 /**
@@ -134,6 +134,16 @@ const cssBanner = styled('div', `
   }
 `);
 
+export const cssBannerLink = styled('span', `
+  cursor: pointer;
+  color: unset;
+  text-decoration: underline;
+
+  &:hover, &:focus {
+    color: unset;
+  }
+`);
+
 const cssButtons = styled('div', `
   display: flex;
   gap: 16px;