Re-derive the preview source at the crop's own resolution

A committed crop kept only its share of the 1600px whole-photo copy, so the
stage then showed a 2x upscale and the photo read as broken. While a crop is
live the source is remade 1/f larger (f = the crop's longest side as a
fraction of the photo's), capped so the decode stays bounded; UNDO/CANCEL
and a new photo put the 1600px copy back.

docker/frontend/src/App.tsx

@@ -30,6 +30,12 @@ import type { MsgKey } from './i18n/vi';
 // Preview renders on a downscaled copy; export always re-renders the original
 // bytes, so nothing shown here is baked into the file.
 const PREVIEW_MAX = 1600;
+// A committed crop keeps only a fraction of the photo, so the 1600px copy
+// leaves too few pixels behind and the result reads as blocky once the stage
+// scales it up. While a crop is live the source is re-derived larger, inverted
+// by that fraction, and capped here so the decode stays bounded.
+const CROP_PREVIEW_MAX = 3200;
+const CROP_PREVIEW_MIN_FRAC = 0.05;
 const PREVIEW_DEBOUNCE_MS = 120;
 
 const FRAMES: { id: FrameId; label: MsgKey }[] = [
@@ -166,6 +172,27 @@ export function Workspace() {
   const cropEditing = crop !== 'none' && !cropApplied;
   const cropLive = crop !== 'none' && cropApplied;
   const cropWH = crop !== 'none' && crop !== 'free' ? CROP_W_H[crop] : null;
+  // A committed crop keeps only part of the photo, so the 1600px copy leaves
+  // too few pixels behind and the stage shows a blocky upscale. While the crop
+  // is live the source is re-derived larger — 1/f, where f is the crop's
+  // longest side as a fraction of the photo's, capped so the decode stays
+  // bounded. cropRect is in the rotated frame, so the shape is swapped first.
+  const cropLongFrac = (() => {
+    const d = photoDims;
+    const turn = rotation % 180 !== 0;
+    const f = Math.max(
+      (turn ? d?.h ?? 0 : d?.w ?? 0) * cropRect.w,
+      (turn ? d?.w ?? 0 : d?.h ?? 0) * cropRect.h
+    );
+    const long = Math.max(d?.w ?? 0, d?.h ?? 0);
+    return long > 0 ? f / long : Math.min(cropRect.w, cropRect.h);
+  })();
+  const previewMax = cropLive
+    ? Math.min(
+        CROP_PREVIEW_MAX,
+        Math.max(PREVIEW_MAX, Math.round(PREVIEW_MAX / Math.max(cropLongFrac, CROP_PREVIEW_MIN_FRAC)))
+      )
+    : PREVIEW_MAX;
 
   // --- undo -----------------------------------------------------------------
   // The stack holds whole looks. remember() reads a ref written during render,
@@ -224,6 +251,10 @@ export function Workspace() {
     (new URLSearchParams(window.location.search).has('auth') ? 'signup' : null));
 
   const renderToken = useRef(0);
+  // Which previewMax the bytes in previewBytes were made at, so the upgrade
+  // effect does not redo the load-time 1600px copy.
+  const previewMaxRef = useRef(PREVIEW_MAX);
+  const previewJobRef = useRef(0);
 
   // --- boot -----------------------------------------------------------------
   useEffect(() => {
@@ -308,6 +339,20 @@ export function Workspace() {
     // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [engine, previewBytes, recipe, frameId, cropEditing, cropLive, cropRect, rotation, useGeotag, gps, gpsStyle, markOn, markText, markStyle]);
 
+  // The source copy behind previewBytes, re-made only when previewMax moves:
+  // applying a crop upgrades it, CANCEL/UNDO brings the 1600px one back.
+  useEffect(() => {
+    if (!source || previewMaxRef.current === previewMax) return;
+    const job = ++previewJobRef.current;
+    resizedJpeg(source.bytes, previewMax)
+      .then((bytes) => {
+        if (job !== previewJobRef.current) return;
+        previewMaxRef.current = previewMax;
+        setPreviewBytes(bytes);
+      })
+      .catch(() => undefined);
+  }, [source, previewMax]);
+
   // --- input ---------------------------------------------------------------
   // One photo, two ways in: a drop/pick (which also stores it for the next
   // visit) and the session restore, whose params are already in state and so
@@ -330,6 +375,9 @@ export function Workspace() {
         if (prev) URL.revokeObjectURL(prev.url);
         return { bytes, name, url: URL.createObjectURL(blob) };
       });
+      // The new photo's copy is the 1600px one; a live crop then upgrades it.
+      previewMaxRef.current = PREVIEW_MAX;
+      previewJobRef.current++;
       setPreviewBytes(preview);
       setGps(found);
       // A new photo starts a new history whose floor is the default look, so