feat: App mode - Rework save flow (#10439)

## Summary

Users were finding the final step of the builder flow
confusing/misleading, with the "choose default mode" not actually saving
the workflow and people losing changes. This updates it to remove
"save"/"set default" as a step in the builder, and changes it to a
distinct action.

## Changes

- **What**: 
- add mode selection tab on footer toolbar
- extract reusable radio group component
- remove setting default mode dialog
- add save/save as/saved dialogs

## Screenshots (if applicable)


https://github.com/user-attachments/assets/c7439c2e-a917-4f2b-b176-f8bb8c10026d

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-10439-feat-App-mode-Rework-save-flow-32d6d73d3650814781b6c7bbea685a97)
by [Unito](https://www.unito.io)

packages/design-system/src/css/style.css

@@ -233,6 +233,7 @@
   --interface-builder-mode-background: var(--color-ocean-300);
   --interface-builder-mode-button-background: var(--color-ocean-600);
   --interface-builder-mode-button-foreground: var(--color-white);
+  --interface-builder-mode-footer-background: var(--color-ocean-900);
 
   --nav-background: var(--color-white);
 
@@ -376,6 +377,7 @@
   --interface-builder-mode-background: var(--color-ocean-900);
   --interface-builder-mode-button-background: var(--color-ocean-600);
   --interface-builder-mode-button-foreground: var(--color-white);
+  --interface-builder-mode-footer-background: var(--color-ocean-900);
 
   --nav-background: var(--color-charcoal-800);
 
@@ -519,6 +521,9 @@
   --color-interface-builder-mode-button-foreground: var(
     --interface-builder-mode-button-foreground
   );
+  --color-interface-builder-mode-footer-background: var(
+    --interface-builder-mode-footer-background
+  );
   --color-interface-stroke: var(--interface-stroke);
   --color-nav-background: var(--nav-background);
   --color-node-border: var(--node-border);