fix: Improve redirect error message (#4831)

## Description


[Discussion](https://discord.com/channels/955905230107738152/1334938982882082847)

<img width="426" alt="image"
src="https://github.com/user-attachments/assets/0816820d-c2be-4666-80f9-74eb7af2b169"
/>


## Steps for reproduction

1. click button
2. expect xyz

## Code Review

- [ ] hi @kof, I need you to do
  - conceptual review (architecture, feature-correctness)
  - detailed review (read every line)
  - test it on preview

## Before requesting a review

- [ ] made a self-review
- [ ] added inline comments where things may be not obvious (the "why",
not "what")

## Before merging

- [ ] tested locally and on preview environment (preview dev login:
0000)
- [ ] updated [test
cases](https://github.com/webstudio-is/webstudio/blob/main/apps/builder/docs/test-cases.md)
document
- [ ] added tests
- [ ] if any new env variables are added, added them to `.env` file

Author: kof

packages/sdk/src/schema/pages.ts

@@ -92,7 +92,10 @@ export const PagePath = z
   .string()
   .refine((path) => path !== "", "Can't be empty")
   .refine((path) => path !== "/", "Can't be just a /")
-  .refine((path) => path === "" || path.startsWith("/"), "Must start with a /")
+  .refine(
+    (path) => path === "" || path.startsWith("/"),
+    "Must start with a / or a full URL e.g. https://website.org"
+  )
   .refine((path) => path.endsWith("/") === false, "Can't end with a /")
   .refine((path) => path.includes("//") === false, "Can't contain repeating /")
   .refine(