fix(react-ui/e2e): scope backendTrigger to <main> so it skips LanguageSwitcher

The LanguageSwitcher added in the i18n PR (#9642) lives in the sidebar
and also uses aria-haspopup="listbox" — same attribute the import-form
SearchableSelect uses. The Batch D / E tests' helper resolved the trigger
with `page.locator('button[aria-haspopup="listbox"]').first()`, which now
returns the language switcher (rendered first in DOM order, in the
sidebar) instead of the backend dropdown.

After clicking the wrong button, getByRole('option', { name: 'llama-cpp' })
naturally never resolves — language options aren't backend names — and
the test times out at 30s.

Scope the locator to the <main className="main-content"> wrapper so only
buttons inside the route's main content area match. The page layout has
the Sidebar outside <main>, so this cleanly excludes it.

Assisted-by: Claude:claude-opus-4-7[1m] [Claude Code]
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>