fix(next): login redirect crashes page (#13786)

## Problem

When logging in with a `?redirect=`, the target page may crash. This
happens because the update from `SyncClientConfig` is applied in a
`useEffect`, which runs **_after_** the target page's client components
render. As a result, those components read the unauthenticated client
config on first render - even though they expect the authenticated one.

## Steps To Reproduce

1. logout
2. login with ?redirect=
3. document client component incorrectly still receives unauthenticated
client config on render
4. THEN the SyncClientConfig useEffect runs. Too late
5. Potential error (depending on the page, e.g. document view) -
document client component expects sth to be there, but it is not

## Solution

This PR replaces `SyncClientConfig` with a `RootPageConfigProvider`.
This new provider shadows the root layout’s `ConfigProvider` and ensures
the correct client config is available **_immediately_**.

It still updates the config using `useEffect` (the same
way`SyncClientConfig` did), but with one key difference:

- During the brief window between the redirect and the effect running,
it overrides the root layout’s config and provides the fresh,
authenticated config from the root page via the `RootConfigContext`.

This guarantees that client components on the target page receive the
correct config on first render, preventing errors caused by reading the
outdated unauthenticated config.

## Additional change - get rid of `UnsanitizedClientConfig` and
`sanitizeClientConfig`

Those functions added unnecessary complexity, just to build the
blocksMap. I removed those and perform the building of the `blocksMap`
server-side - directly in `createClientConfig`.

---
- To see the specific tasks where the Asana app for GitHub is being
used, see below:
  - https://app.asana.com/0/0/1211334752795621

test/auth/e2e.spec.ts

@@ -52,6 +52,7 @@ describe('Auth', () => {
     page = await context.newPage()
     initPageConsoleErrorCatch(page)
   })
+
   describe('create first user', () => {
     beforeAll(async () => {
       await reInitializeDB({
@@ -386,6 +387,36 @@ describe('Auth', () => {
 
         await saveDocAndAssert(page)
       })
+
+      test('ensure login page with redirect to users document redirects properly after login, without client error', async () => {
+        await page.goto(url.admin)
+
+        await page.goto(`${serverURL}/admin/logout`)
+
+        await expect(page.locator('.login')).toBeVisible()
+
+        const users = await payload.find({
+          collection: slug,
+          limit: 1,
+        })
+        const userDocumentRoute = `${serverURL}/admin/collections/users/${users?.docs?.[0]?.id}`
+
+        await page.goto(userDocumentRoute)
+
+        await expect(page.locator('#field-email')).toBeVisible()
+        await expect(page.locator('#field-password')).toBeVisible()
+
+        await page.locator('.form-submit > button').click()
+
+        // Expect to be redirected to the correct page
+        await expect
+          .poll(() => page.url(), { timeout: POLL_TOPASS_TIMEOUT })
+          .toBe(userDocumentRoute)
+
+        // Previously, this would crash the page with a "Cannot read properties of undefined (reading 'match')" error
+
+        await expect(page.locator('#field-roles')).toBeVisible()
+      })
     })
   })
 })