feat(frontend): visual QA pass — unified page titles, CTAs, native controls, 404 fixes

- add PageTitle and apply it on /download, /themes, /configs, /showcase, /link, 404
- reset range-slider thumbs in the theme editor; fold color picker into the swatch
- hide horizontal scrollbars in the ClickGui mock; clip html to kill 100vw pan
- pin the footer under short pages via flex AppShell; split 404 art from caption
- one landing CTA geometry (lv-cta-lg / quiet); CLOUD eyebrow; auth caption scrim
- showcase error hides sort/pager; query retry limited to 1

frontend/src/shared/ui/tests/ui.test.tsx

@@ -2,6 +2,7 @@ import { render, screen } from '@testing-library/react'
 import userEvent from '@testing-library/user-event'
 import { expect, test, vi } from 'vitest'
 import { Button } from '../Button'
+import { PageTitle } from '../PageTitle'
 import { TextField } from '../TextField'
 import { ShareCode } from '../ShareCode'
 import { Notice } from '../Notice'
@@ -34,3 +35,11 @@ test('error notice is an alert', () => {
   render(<Notice tone="error">Неверный код</Notice>)
   expect(screen.getByRole('alert')).toHaveTextContent('Неверный код')
 })
+
+test('page title renders one h1 with an optional lead', () => {
+  render(
+    <PageTitle lead="Короткое описание">Мои конфиги</PageTitle>,
+  )
+  expect(screen.getByRole('heading', { level: 1, name: 'Мои конфиги' })).toBeInTheDocument()
+  expect(screen.getByText('Короткое описание')).toBeInTheDocument()
+})