Fix preview cutoff + sticky left panel

1. Preview endpoint now supports HTTP Range requests (HTTP 206 Partial)
   - HTML5 video player needs Range support to seek/buffer properly
   - Without it, video would cut off after a few seconds
   - Returns chunks of 64KB on demand

2. Left panel (upload form) is now sticky (position: sticky)
   - Stays in view while right panel (jobs list) scrolls
   - On mobile (<800px) reverts to normal flow

templates/index.html

@@ -56,9 +56,20 @@
       display: grid;
       grid-template-columns: 1fr 1fr;
       gap: 24px;
+      align-items: start;
+    }
+    main > section.card:first-of-type {
+      position: sticky;
+      top: 16px;
+      max-height: calc(100vh - 32px);
+      overflow-y: auto;
     }
     @media (max-width: 800px) {
       main { grid-template-columns: 1fr; }
+      main > section.card:first-of-type {
+        position: static;
+        max-height: none;
+      }
     }
     .card {
       background: var(--panel);