fix(login): language switcher layout and button colours on the login screen

[ogorzalka]

Problem

On a dressed login screen (a theme with config/login.php) and a site with a second language installed:

• the language switcher's label ("Language" and its icon) sat at the left edge of the window, far from its dropdown;
• the switcher's Change button kept wp-admin's blue instead of the theme's primary colour.

Cause

• .pollora-login form { display: flow-root }, meant for the login card, also reached #language-switcher. WordPress makes that form an inline-block so its left-aligned label sits above the dropdown; as a block it spanned the page.
• The button rules were written .pollora-login .wp-core-ui … (descendant), but both classes are on <body>, so they never matched. The primary button was still styled through its second selector (.pollora-login .button-primary), minus the padding WordPress's .button-large rule kept; the secondary button rule had no fallback.

Fix

• .pollora-login .language-switcher form { display: inline-block; }
• .pollora-login .wp-core-ui → .pollora-login.wp-core-ui (compound).

Verified on pollora-test (WordPress 7.1.2, fr_FR + en_US, theme apiary) at 1280 px and 360 px: label and dropdown share the same left edge, the button takes the theme's primary colour, the submit button keeps its gradient and colours (slightly wider: its own padding now applies).

The E2E login test now checks, when the switcher is present, that its label is aligned with the dropdown.