fix(intro): splash always honors App's decision + video streams 3x lighter

- SplashScreen no longer re-checks neode_intro_seen itself: it silently
  skipped the whole logo-tap/typing sequence for any browser that had
  seen the intro before, even when App.vue explicitly requested it (demo
  fresh visits, Replay Intro). App.vue is the single decision point.
- Intro video re-encoded 12.7MB/12.6Mbps → 4.3MB/4.2Mbps (1920x1080 CRF23
  faststart, muted track dropped) — visually identical on this
  illustrated content; cache-buster bumped to v=8.
- Video is <link rel=preload>ed the moment the splash is scheduled (the
  <video> element only mounts ~20s in, so cold caches used to start the
  download mid-sequence and stutter).
- nginx demo cache regex now covers mp4/webm/mp3/webp/woff2 so repeat
  visits play from browser cache.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

neode-ui/docker/nginx-demo.conf

@@ -187,8 +187,9 @@ http {
             try_files $uri $uri/ /index.html;
         }
 
-        # Cache static assets
-        location ~* \.(js|css|png|jpg|jpeg|gif|ico|svg)$ {
+        # Cache static assets (media too — the intro video/audio are versioned
+        # with ?v=N query busters, so immutable is safe)
+        location ~* \.(js|css|png|jpg|jpeg|gif|ico|svg|webp|mp4|webm|mp3|woff2?)$ {
             expires 1y;
             add_header Cache-Control "public, immutable";
         }