fix(wm): let the library editor pinch a watermark mark

onLibTouchStart only recorded the pinch baseline when both fingers arrived in
the same touch-start, so a second finger landing later (the normal case) left
wmPinchRef null and the move handler scaled nothing -- pinch-the-mark worked
on the camera viewfinder but not on a library photo. Seed the baseline in the
move handler, as the camera path already does.

src/components/Viewfinder.tsx

@@ -1349,7 +1349,25 @@ const Viewfinder = forwardRef<ViewfinderHandle, ViewfinderProps>(function Viewfi
     // WATERMARK tab open: two fingers scale the mark under them (or do
     // nothing) — the photo itself only zooms once the editor is closed.
     if (ts.length >= 2 && wmEditing) {
-      const p = wmPinchRef.current;
+      // The second finger normally lands on its own, so the START handler never
+      // saw the pair and wmPinchRef is still null -- which is why pinching a
+      // mark worked on the camera (that path seeds here) but not in the library
+      // editor. Seed the baseline the same way.
+      let p = wmPinchRef.current;
+      if (!p) {
+        const tgt = wmTargetAt(
+          (ts[0].locationX + ts[1].locationX) / 2,
+          (ts[0].locationY + ts[1].locationY) / 2
+        );
+        if (tgt) {
+          p = {
+            tgt,
+            d: Math.hypot(ts[0].locationX - ts[1].locationX, ts[0].locationY - ts[1].locationY),
+            size: wmSizeOf(tgt),
+          };
+          wmPinchRef.current = p;
+        }
+      }
       if (p) {
         const d = Math.hypot(ts[0].locationX - ts[1].locationX, ts[0].locationY - ts[1].locationY);
         applyWmScale(p.tgt, (p.size * d) / Math.max(p.d, 1));