fix(web): lock body scroll while modal is open

Native <dialog> in iOS Safari (and a few other browsers) doesn't
prevent the page underneath from scrolling when the user scrolls
inside the dialog or near its edges. Save and restore body overflow
on open/close so the background stays put. Stays correct for stacked
dialogs because we save the previous value rather than blanket-reset
to ''.

web/components/confirm-dialog.tsx

@@ -41,6 +41,19 @@ export default function ConfirmDialog({
     }
   }, [open]);
 
+  // Lock body scroll while open. Native <dialog> doesn't do this in all
+  // browsers (notably iOS Safari), so background scroll can leak through
+  // when scrolling on the dialog content. We restore the previous value
+  // on close so other modals stacking later don't regress this.
+  useEffect(() => {
+    if (!open) return;
+    const previous = document.body.style.overflow;
+    document.body.style.overflow = "hidden";
+    return () => {
+      document.body.style.overflow = previous;
+    };
+  }, [open]);
+
   return (
     <dialog
       ref={ref}

web/components/form-dialog-shell.tsx

@@ -40,6 +40,16 @@ export default function FormDialogShell({
     else if (!open && dialog.open) dialog.close();
   }, [open]);
 
+  // Lock body scroll while open (native <dialog> doesn't on iOS Safari).
+  useEffect(() => {
+    if (!open) return;
+    const previous = document.body.style.overflow;
+    document.body.style.overflow = "hidden";
+    return () => {
+      document.body.style.overflow = previous;
+    };
+  }, [open]);
+
   return (
     <dialog
       ref={ref}