Fix drone dark mode regression and add devkit video

- Apply filter:invert(1) to the SVG element directly instead of the wrapper div,
  with explicit background:#000 on the container in dark mode — more reliable than
  inverting the whole div and avoids edge cases with the white background trick
- Replace devkits img-slot placeholder with droneRun.webm autoplay loop video

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

style.css

@@ -257,7 +257,8 @@ img { display: block; max-width: 100%; }
   height: clamp(360px, 48vw, 640px);
 }
 
-html.dark .drone-vision-bg { filter: invert(1); }
+html.dark .drone-vision-bg { background: #000; }
+html.dark .drone-vision-bg svg { filter: invert(1); }
 
 .hero-caps-strip {
   background: rgba(255,255,255,0.86);