perf(preview): debounce full-res /proxy preload by 400ms

Rapid arrow-nav was firing one /proxy fetch per photo with no way to
abort (new Image() has no abort). Holding the right arrow through ten
photos in two seconds left ten multi-MB transfers in flight competing
for bandwidth and the RAW/HEIC transcoder. Now the preload only kicks
in if the user lingers on a photo for 400ms; otherwise the timer is
cleared and no /proxy request is made.

frontend/src/components/preview/PreviewImage.tsx

@@ -59,21 +59,30 @@ function PreviewStillImage({ photo }: { photo: Photo }) {
   const fullResSrc = getPreviewFullResSrc(photo)
   const src = usingFullRes ? fullResSrc : thumbSrc
 
-  // Reset everything when the photo changes, then start the background
+  // Reset everything when the photo changes, then queue a *debounced*
-  // full-res preload. Cancel the preloader's callback on unmount/change so a
+  // background full-res preload. Debouncing matters because `new Image()`
-  // late-arriving onload from the previous photo can't flip state for the
+  // requests can't be aborted: rapid arrow-nav would otherwise leave a
-  // current one.
+  // dozen multi-MB /proxy fetches in flight, saturating the user's
+  // bandwidth (and the backend's transcoder for RAW/HEIC) for photos the
+  // user already navigated past. Only photos the user lingers on for
+  // FULL_RES_PRELOAD_DELAY_MS trigger the /proxy fetch.
   useEffect(() => {
     setLoaded(false)
     setUsingFullRes(false)
     setScale(1)
     setOffset({ x: 0, y: 0 })
 
-    const preloader = new Image()
+    const FULL_RES_PRELOAD_DELAY_MS = 400
-    preloader.onload = () => setUsingFullRes(true)
+    let preloader: HTMLImageElement | null = null
-    preloader.src = fullResSrc
+    const timer = window.setTimeout(() => {
+      preloader = new Image()
+      preloader.onload = () => setUsingFullRes(true)
+      preloader.src = fullResSrc
+    }, FULL_RES_PRELOAD_DELAY_MS)
+
     return () => {
-      preloader.onload = null
+      window.clearTimeout(timer)
+      if (preloader) preloader.onload = null
     }
   }, [photo.id, fullResSrc])