fix(ui): drop stray tab border (#51657)

## Problem

there's a legacy border style duplicating border after recent rabs
change in docs

## Solution

- removes stray border

| state | preview |
| -------|------|
| before | <img width="913" height="509" alt="image"
src="https://github.com/user-attachments/assets/dba5a5ee-5212-4be2-86c0-e96d9dac41fb"
/> |
| after | <img width="913" height="509" alt="image"
src="https://github.com/user-attachments/assets/1f7d20eb-2f03-45b0-9a2c-4af8fa335784"
/> |

<!--
## Preview links

If relevant, include links to changed pages for easy review access.

Copy the preview base URL from the Vercel bot comment on this PR. Use
the following table as an example template.

| Site | Live | Preview | Search for |
| -------------- |
-------------------------------------------------------------------------
|
------------------------------------------------------------------------------------------------------------
| ----------------------------- |
| WWW | [/blog/your-post](https://supabase.com/blog/your-post) |
[/blog/your-post](https://zone-www-dot-com-git-branch-name-supabase.vercel.app/blog/your-post)
| unique phrase from the change |
| Docs |
[/docs/guides/your-page](https://supabase.com/docs/guides/your-page) |
[/docs/guides/your-page](https://docs-git-branch-name-supabase.vercel.app/docs/guides/your-page)
| unique phrase from the change |
| Studio | [/dashboard](https://supabase.com/dashboard) |
[/dashboard](https://studio-git-branch-name-supabase.vercel.app/dashboard)
| unique phrase from the change |
| Design system | [/design-system](https://supabase.com/design-system) |
[/design-system](https://design-system-git-branch-name-supabase.vercel.app/design-system)
| unique phrase from the change |
| UI library | [/library](https://supabase.com/library) |
[/library](https://ui-library-git-branch-name-supabase.vercel.app/library)
| unique phrase from the change |
| Knowledge base |
[/kb/guides/your-page](https://supabase.com/kb/guides/your-page) |
[/kb/guides/your-page](https://kb-git-branch-name-supabase.vercel.app/kb/guides/your-page)
| unique phrase from the change |
-->

<!-- ## Additional context

Optionally add any other context or screenshots.

-->

## Review instructions

1. visit
`/docs/guides/auth/passwords#signing-up-with-an-email-and-password`

## Checklist

Check all before review:

- [x] I have read
[CONTRIBUTING.md](https://github.com/supabase/supabase/blob/master/CONTRIBUTING.md)
- [x] If I wrote a new docs topic or edited an existing topic, I used
the `/write-the-docs` or `/edit-the-docs` skill, which applies the docs
[style
guide](https://github.com/supabase/supabase/tree/master/apps/docs/style-guide)

apps/docs/content/guides/auth/passwords.mdx

@@ -30,7 +30,6 @@ The instructions in this section assume that email confirmations are enabled.
     style: {
       top: 'var(--header-height)',
       maskImage: 'none',
-      borderBottom: '1px solid oklch(from var(--border-default) l c h / 1)',
     }
   }}
   size="large"