ui(preview): load 1280px thumb first, upgrade to full-res in bg

The /proxy endpoint is slow on first hit, especially for RAW/HEIC where it
transcodes synchronously. Preview now renders the pre-generated large thumb
immediately, then preloads /proxy via Image() and swaps src when ready, so
zoom (Z key / wheel) still reaches the original pixels.

frontend/src/components/preview/PreviewImage.tsx

@@ -3,7 +3,7 @@ import { useHotkeys } from 'react-hotkeys-hook'
 import type { Photo } from '../../types/photo'
 import {
   getPreviewImageSrc,
-  getPreviewFallbackSrc,
+  getPreviewFullResSrc,
   getVideoSrc,
   isVideo,
 } from './previewSrc'
@@ -44,7 +44,9 @@ function PreviewVideo({ photo }: { photo: Photo }) {
 
 function PreviewStillImage({ photo }: { photo: Photo }) {
   const [loaded, setLoaded] = useState(false)
-  const [usingFallback, setUsingFallback] = useState(false)
+  // Progressive enhancement: render the 1280px thumb first, then preload the
+  // full-res /proxy in the background and flip this to true once decoded.
+  const [usingFullRes, setUsingFullRes] = useState(false)
 
   // scale=1 means "fit to viewport". Anything >1 zooms in; we don't allow <1
   // because the fit size already fills the viewport.
@@ -53,21 +55,35 @@ function PreviewStillImage({ photo }: { photo: Photo }) {
   const dragStateRef = useRef<{ x: number; y: number; ox: number; oy: number } | null>(null)
   const imgRef = useRef<HTMLImageElement>(null)
 
-  // Reset everything when the photo changes.
+  const thumbSrc = getPreviewImageSrc(photo)
+  const fullResSrc = getPreviewFullResSrc(photo)
+  const src = usingFullRes ? fullResSrc : thumbSrc
+
+  // Reset everything when the photo changes, then start the background
+  // full-res preload. Cancel the preloader's callback on unmount/change so a
+  // late-arriving onload from the previous photo can't flip state for the
+  // current one.
   useEffect(() => {
     setLoaded(false)
-    setUsingFallback(false)
+    setUsingFullRes(false)
     setScale(1)
     setOffset({ x: 0, y: 0 })
-  }, [photo.id])
 
-  const primarySrc = getPreviewImageSrc(photo)
-  const fallbackSrc = getPreviewFallbackSrc(photo)
-  const src = usingFallback ? fallbackSrc : primarySrc
+    const preloader = new Image()
+    preloader.onload = () => setUsingFullRes(true)
+    preloader.src = fullResSrc
+    return () => {
+      preloader.onload = null
+    }
+  }, [photo.id, fullResSrc])
 
+  // If the thumb 404s (e.g. derivative not yet generated for a fresh import),
+  // jump straight to the full-res — the same <img> element will retry against
+  // /proxy. If /proxy also fails, the browser shows its broken-image icon and
+  // we surface no further fallback.
   const handleError = () => {
-    if (!usingFallback && primarySrc !== fallbackSrc) {
-      setUsingFallback(true)
+    if (!usingFullRes) {
+      setUsingFullRes(true)
     }
   }
 
@@ -150,7 +166,7 @@ function PreviewStillImage({ photo }: { photo: Photo }) {
     >
       <img
         ref={imgRef}
-        key={`${photo.id}-${usingFallback}`}
+        key={photo.id}
         src={src}
         alt={photo.filename}
         loading="eager"

frontend/src/components/preview/previewSrc.ts

@@ -10,22 +10,29 @@ export function isVideo(photo: Photo): boolean {
 }
 
 /**
- * Pick the best display URL for a still photo in the preview view.
+ * Initial display URL for a still photo in the preview view: the 1280px
+ * `large` WebP thumbnail. Cheap and always pre-generated, so the preview
+ * appears instantly even for RAW/HEIC photos whose full-res /proxy would
+ * otherwise transcode synchronously on first hit.
  *
- * Always uses the /proxy endpoint, which the backend resolves to:
- *   - the original file for web-safe formats (JPEG/PNG/WebP/GIF)
- *   - a transcoded full-res WebP for RAW/HEIC/TIFF (cached on first hit)
+ * Pair with `getPreviewFullResSrc` for progressive enhancement: load this
+ * first, then preload the full-res in the background and swap it in.
  *
  * Videos go through `getVideoSrc` instead and use /original directly.
  */
 export function getPreviewImageSrc(photo: Photo): string {
-  return photosApi.getProxyUrl(photo.id)
+  return photosApi.getThumbnailUrl(photo.id, 'large')
 }
 
-/** Fallback used when the proxy endpoint fails or 404s — shows the 1280px
- *  large thumbnail so the user still sees something. */
-export function getPreviewFallbackSrc(photo: Photo): string {
-  return photosApi.getThumbnailUrl(photo.id, 'large')
+/**
+ * Full-resolution URL fetched in the background after the thumbnail is
+ * already visible. The /proxy endpoint serves the original file for
+ * web-safe formats (JPEG/PNG/WebP/GIF) or a transcoded full-res WebP for
+ * RAW/HEIC/TIFF (cached on first hit). Used so Z-key / wheel zoom stays
+ * pixel-sharp once the upgrade lands.
+ */
+export function getPreviewFullResSrc(photo: Photo): string {
+  return photosApi.getProxyUrl(photo.id)
 }
 
 export function getVideoSrc(photo: Photo): string {