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/pages/public/NotFoundPage.tsx

@@ -1,15 +1,14 @@
 import { useTranslation } from 'react-i18next'
 import { Link } from 'react-router'
-import { NOT_FOUND_ART } from '../../shared/easter/art.generated'
+import { BreakableBlock } from '../../shared/easter/BreakableBlock'
 import { PageTitle } from '../../shared/layout/PageTitle'
 
 export default function NotFoundPage() {
   const { t } = useTranslation('common')
   return (
     <section className="flex flex-1 flex-col items-start gap-6 py-16">
-      <pre aria-hidden="true" className="w-fit max-w-full overflow-x-auto rounded-panel border border-white/10 bg-black/30 p-4 font-code text-xs leading-tight text-ice">
-{`${NOT_FOUND_ART}\n`}{t('notFound.console')}
-      </pre>
+      <BreakableBlock />
+      <p className="max-w-md text-sm text-frost">{t('notFound.blockHint')}</p>
       <PageTitle
         lead={
           <>

frontend/src/shared/easter/BreakableBlock.tsx

@@ -0,0 +1,71 @@
+import { AnimatePresence, motion, useReducedMotion } from 'motion/react'
+import { useState } from 'react'
+import { useTranslation } from 'react-i18next'
+import './block.css'
+
+/** Clicks needed before the block shatters: two crack stages then the burst. */
+const BREAK_AT = 3
+
+type Particle = { id: number; dx: string; dy: string }
+
+/**
+ * `/` 404 easter egg: a grass block drops into a dusk world, cracks over a few clicks,
+ * then bursts into square debris and respawns. Under reduced motion it skips the fall and
+ * the burst — clicking still walks the crack stages and resets.
+ */
+export function BreakableBlock() {
+  const { t } = useTranslation('common')
+  const reduced = useReducedMotion()
+  const [cracks, setCracks] = useState(0)
+  const [particles, setParticles] = useState<Particle[]>([])
+  const [cycle, setCycle] = useState(0) // bump to replay the drop after a burst
+
+  function burst() {
+    setParticles(
+      Array.from({ length: 10 }, (_, i) => ({
+        id: i,
+        dx: `${(i - 4.5) * 16}px`,
+        dy: `${-40 - (i % 3) * 24}px`,
+      })),
+    )
+    setCracks(0)
+    setCycle((c) => c + 1)
+    window.setTimeout(() => setParticles([]), 500)
+  }
+
+  function crack() {
+    const next = cracks + 1
+    if (next >= BREAK_AT) {
+      burst()
+      return
+    }
+    setCracks(next)
+  }
+
+  return (
+    <div className="lv-block-world">
+      <motion.span
+        key={cycle}
+        className="lv-block-wrap"
+        initial={reduced ? false : { y: -220, opacity: 0 }}
+        animate={{ y: 0, opacity: 1 }}
+        transition={reduced ? { duration: 0 } : { type: 'spring', stiffness: 320, damping: 12 }}
+      >
+        <button type="button" className="lv-block" aria-label={t('notFound.blockAria')} onClick={crack}>
+          {cracks > 0 && <span className="lv-block-crack" data-stage={String(Math.min(cracks, 2))} aria-hidden="true" />}
+        </button>
+      </motion.span>
+
+      <AnimatePresence>
+        {particles.map((p) => (
+          <motion.span
+            key={`${cycle}-${p.id}`}
+            className="lv-block-particle"
+            style={{ ['--dx' as string]: p.dx, ['--dy' as string]: p.dy }}
+            exit={{ opacity: 0 }}
+          />
+        ))}
+      </AnimatePresence>
+    </div>
+  )
+}

frontend/src/shared/easter/block.css

@@ -0,0 +1,87 @@
+/* 404 easter egg: a dusk world with a grass block that falls, cracks and bursts. */
+.lv-block-world {
+  position: relative;
+  display: grid;
+  place-items: end center;
+  width: 100%;
+  max-width: 420px;
+  height: 220px;
+  overflow: hidden;
+  border: 1px solid rgb(255 255 255 / 0.1);
+  border-radius: var(--radius-panel, 16px);
+  background: linear-gradient(180deg, #10162e 0%, #1a2340 55%, #24314f 100%);
+}
+
+.lv-block-wrap {
+  margin-bottom: 26px;
+}
+
+/* The block itself: grass crown over dirt, pixel-inset edges like a Minecraft tile. */
+.lv-block {
+  position: relative;
+  display: block;
+  width: 84px;
+  height: 84px;
+  padding: 0;
+  border: 0;
+  cursor: pointer;
+  image-rendering: pixelated;
+  background: linear-gradient(#6c4b2a 0, #6c4b2a 100%);
+  box-shadow:
+    inset 0 0 0 3px rgb(0 0 0 / 0.22),
+    inset 0 -10px 0 rgb(0 0 0 / 0.12);
+}
+.lv-block::before {
+  content: '';
+  position: absolute;
+  inset: 0 0 auto 0;
+  height: 30%;
+  background: linear-gradient(#7ede68 0, #4f9c42 100%);
+  box-shadow: inset 0 -3px 0 rgb(0 0 0 / 0.18);
+}
+.lv-block:focus-visible {
+  outline: 2px solid var(--color-ice, #5cc8e7);
+  outline-offset: 3px;
+}
+
+/* Crack overlay: dark jagged strokes revealed stage by stage. */
+.lv-block-crack {
+  position: absolute;
+  inset: 0;
+  pointer-events: none;
+  background-repeat: no-repeat;
+  background-image:
+    linear-gradient(65deg, transparent 47%, rgb(0 0 0 / 0.6) 48% 52%, transparent 53%),
+    linear-gradient(-40deg, transparent 44%, rgb(0 0 0 / 0.6) 45% 49%, transparent 50%),
+    linear-gradient(15deg, transparent 51%, rgb(0 0 0 / 0.6) 52% 55%, transparent 56%);
+}
+.lv-block-crack[data-stage='1'] { background-image: linear-gradient(65deg, transparent 47%, rgb(0 0 0 / 0.6) 48% 52%, transparent 53%); }
+.lv-block-crack[data-stage='2'] {
+  background-image:
+    linear-gradient(65deg, transparent 47%, rgb(0 0 0 / 0.6) 48% 52%, transparent 53%),
+    linear-gradient(-40deg, transparent 44%, rgb(0 0 0 / 0.6) 45% 49%, transparent 50%);
+}
+
+/* Square debris that flies out and fades when the block bursts. */
+.lv-block-particle {
+  position: absolute;
+  left: 50%;
+  bottom: 26px;
+  width: 10px;
+  height: 10px;
+  background: #4f9c42;
+  pointer-events: none;
+  animation: lv-block-burst 0.45s ease-out forwards;
+}
+.lv-block-particle:nth-child(even) { background: #6c4b2a; }
+@keyframes lv-block-burst {
+  to {
+    transform: translate(var(--dx), var(--dy)) scale(0.2);
+    opacity: 0;
+  }
+}
+
+@media (prefers-reduced-motion: reduce) {
+  .lv-block-wrap { margin-bottom: 26px; }
+  .lv-block-particle { display: none; }
+}

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)
+})

frontend/src/shared/i18n/common.en.ts

@@ -20,6 +20,8 @@ export const commonEn = {
     body: 'This link may be outdated.',
     home: 'Go home',
     console: '> chunk not found. Try again.',
+    blockAria: 'Break the block',
+    blockHint: 'Click the block — it cracks and bursts, just like in the game.',
   },
   shareCode: {
     copy: 'Copy',

frontend/src/shared/i18n/common.ru.ts

@@ -18,6 +18,8 @@ export const commonRu = {
     body: 'Возможно, ссылка устарела.',
     home: 'На главную',
     console: '> chunk not found. Попробуй ещё раз.',
+    blockAria: 'Сломать блок',
+    blockHint: 'Кликни по блоку — он треснет и рассыплется, как в игре.',
   },
   shareCode: {
     copy: 'Скопировать',