feat(site): loop a faithful copy of the app's recording capsule in the hero

Replace the press-to-play demo and the "see how it works" button with an
example that keeps playing on the right: hold Right Alt, the capsule appears
with live wave bars, elapsed time and the partial transcript, releasing shows
the processing bar, and the cleaned sentence is pasted at the cursor of a
notes window. It cycles through two samples per language.

- Same numbers as apps/desktop recording-tip: 9 bars with cos weights, 100 ms
  updates, 0.5 smoothing, +/-35% jitter, 2-28 px, per-bar colours, 120 px
  progress bar min(95, (1 - 1/(1 + 1.5t)) * 100)%.
- WCAG 2.2.2: a pause button; stops when off screen or the tab is hidden.
  Reduced motion shows one still frame. The animation is aria-hidden with a
  text description for screen readers. Chinese and Japanese transcripts
  appear three characters at a time.
- Add 404.html so unknown paths stop falling back to the landing page, delete
  the duplicate accept-invite.html (/accept-invite/ is canonical) and link it
  straight to /#download, and move the legal pages off the retired orange
  accent (primary button contrast 5.2:1).

site/public/accept-invite/index.html

@@ -67,7 +67,7 @@
 
         <footer class="support-row">
           <span>앱이 설치되지 않았어?</span>
-          <a href="/download.html">안전한 설치 파일 받기</a>
+          <a href="/#download">안전한 설치 파일 받기</a>
         </footer>
       </section>
     </main>