Add video thumbnails and playback in Assets

Video assets now show their first frame as a thumbnail in the grid
instead of a generic Film icon, and the detail modal includes a
video player with controls.

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

client/src/components/AssetCard.jsx

@@ -18,6 +18,8 @@ const formatFileSize = (bytes) => {
 export default function AssetCard({ asset, onClick }) {
   const TypeIcon = typeIcons[asset.type] || File
   const isImage = asset.type === 'image'
+  const isVideo = asset.type === 'video'
+  const hasPreview = (isImage || isVideo) && asset.url
 
   return (
     <div
@@ -36,10 +38,19 @@ export default function AssetCard({ asset, onClick }) {
               e.target.nextSibling.style.display = 'flex'
             }}
           />
+        ) : isVideo && asset.url ? (
+          <video
+            src={asset.url}
+            preload="metadata"
+            muted
+            playsInline
+            className="w-full h-full object-cover pointer-events-none"
+            onLoadedData={(e) => { e.target.currentTime = 0.1 }}
+          />
         ) : null}
         <div
-          className={`flex flex-col items-center justify-center gap-2 ${isImage && asset.url ? 'hidden' : ''}`}
-          style={{ display: isImage && asset.url ? 'none' : 'flex' }}
+          className={`flex flex-col items-center justify-center gap-2 ${hasPreview ? 'hidden' : ''}`}
+          style={{ display: hasPreview ? 'none' : 'flex' }}
         >
           <TypeIcon className="w-10 h-10 text-text-tertiary" />
           <span className="text-xs text-text-tertiary uppercase font-medium">