perf: smoother video player — rAF-driven scrub bar, less cold-start stutter

scrub fill/time were driven off timeupdate, which browsers fire irregularly (a handful of times a second) — visible as the bar stepping instead of gliding. now driven by requestAnimationFrame while playing (display refresh rate), with a direct sync on pause/seeked so it still tracks immediately when not running.\n\npreload=none meant zero data was fetched until the play button was pressed, causing a visible stall right at playback start. preload=metadata lets the browser prime the connection and read the moov atom ahead of time without fetching the full video for visitors who never press play.

layouts/portfolio/single.html

@@ -30,7 +30,7 @@
   <figure class="tui__preview">
     {{ if $video }}
     <div class="player">
-      <video preload="none" playsinline{{ with .Params.thumbnail }} poster="{{ . }}"{{ end }}>
+      <video preload="metadata" playsinline{{ with .Params.thumbnail }} poster="{{ . }}"{{ end }}>
         <source src="{{ $video }}" type="video/mp4">
       </video>
       <button class="player__big" type="button" aria-label="play">▶</button>