feat(frontend): breakable grass block on the 404 page

Implements the last unbuilt easter egg (PLAN Task 11 #5, TODO «Пасхалки» #5): the
404 used to show static ASCII art; now a grass block drops into a dusk world,
cracks over three clicks, then bursts into square debris and respawns. Keyboard
accessible (a real button), and under prefers-reduced-motion the fall and burst
are skipped while the crack stages still work. Replaces the NOT_FOUND_ART pane;
i18n adds notFound.blockAria/blockHint (ru+en).

frontend/src/shared/easter/tests/breakable.test.tsx

@@ -0,0 +1,36 @@
+import { screen } from '@testing-library/react'
+import userEvent from '@testing-library/user-event'
+import { expect, test, vi } from 'vitest'
+import { BreakableBlock } from '../BreakableBlock'
+import { renderApp } from '../../../test/render'
+
+// motion's useReducedMotion reads matchMedia, which jsdom lacks.
+vi.stubGlobal('matchMedia', (query: string) => ({
+  matches: !query.includes('reduce'),
+  media: query,
+  onchange: null,
+  addEventListener() {},
+  removeEventListener() {},
+  addListener() {},
+  removeListener() {},
+  dispatchEvent: () => false,
+}))
+
+test('a block drops in and cracks over three clicks, then bursts and respawns', async () => {
+  const user = userEvent.setup()
+  renderApp([{ path: '/', element: <BreakableBlock /> }])
+
+  // The block remounts (new key) on respawn, so re-query it before each click.
+  const block = () => screen.getByRole('button', { name: 'Сломать блок' })
+  expect(block().querySelector('.lv-block-crack')).toBeNull()
+
+  await user.click(block())
+  expect(block().querySelector('.lv-block-crack')).toHaveAttribute('data-stage', '1')
+  await user.click(block())
+  expect(block().querySelector('.lv-block-crack')).toHaveAttribute('data-stage', '2')
+
+  // Third click shatters: cracks clear (respawn) and debris spawns.
+  await user.click(block())
+  expect(block().querySelector('.lv-block-crack')).toBeNull()
+  expect(document.querySelectorAll('.lv-block-particle').length).toBeGreaterThan(0)
+})