feat(frontend): full redesign of login/register

Owner's complaint: the two-pane layout's decorative right-side panel
("Твои настройки LoVisual на любом компьютере...") made no sense next
to a login form. Dropped the whole two-pane concept, not just its
visuals.

Login/register now render as the mod's own in-game account screen:
night-lake backdrop (the same Backdrop the landing hero uses), a
clock + Вход/Регистрация tab switcher styled like the mod's own
ClickGui tabs, a single glass panel on the mod's own category
colours, and a real mod-style hint row ("X - Y") instead of marketing
copy. What an account actually gets you is stated as plain hints
(4 cloud slots, share a slot's code, the mod signs itself in by code,
no account required to just use the mod) instead of a sales pitch.

- AuthSplit.tsx removed; AuthScreen.tsx + auth.css replace it
- LoginForm/RegisterForm: dropped the redundant login<->register text
  link now that the tab switcher does that job; RegisterForm now
  honours the original `from` redirect (previously always went to
  /configs even when registration started from /link)
- Rewrote auth i18n copy to match the site's established voice

Claude-Session: https://claude.ai/code/session_01F1M1Jic1wTSn4igUENynmZ

frontend/src/features/auth/forms/LoginForm.tsx

@@ -1,6 +1,6 @@
 import { useState, type FormEvent } from 'react'
 import { useTranslation } from 'react-i18next'
-import { Link, useLocation, useNavigate } from 'react-router'
+import { useLocation, useNavigate } from 'react-router'
 import { describeError } from '@shared/api/errors'
 import { Button } from '@shared/ui/Button'
 import { Notice } from '@shared/ui/Notice'
@@ -52,12 +52,6 @@ export function LoginForm() {
       <Button type="submit" busy={busy}>
         {t('login.submit')}
       </Button>
-      <p className="text-sm text-frost">
-        {t('login.noAccount')}{' '}
-        <Link to="/register" className="text-ice underline-offset-4 hover:underline">
-          {t('login.createAccount')}
-        </Link>
-      </p>
     </form>
   )
 }