fix(auth): keep login language switcher clickable

[ppapsmoken]

Problem

The login page language switcher is visible but cannot be clicked reliably because the form layer covers it.

Root cause

The auth layout places the switcher and the later form container at the same z-index (z-10), so the form paints above the switcher.

Change

Raise the switcher wrapper to z-20.

Validation

• bun run typecheck
• ESLint and Prettier
• Desktop and 390x844 local browser checks

Deployment

Console-only UI change; router deployment is not required.