Lightbox: pin and size to visible viewport when framed

The staff-report iframe is expanded to full content height (no nested
scrollbar), so a modal <dialog> centered in its viewport landed in the
middle of the whole document and sat off-screen unless the parent page
was scrolled there. Cross-origin means the iframe can't read the
parent's scroll position itself.

Tab.tpl now broadcasts the iframe's visible slice (webform-mw-viewport,
rAF-throttled on scroll/resize/load and after each height change), and
AttachmentLightbox pins to that slice and fills its height. Falls back
to a fixed box when no viewport message arrives (older extension), and
standalone mode keeps native centering at a taller 85vh.

Extension bumped to 0.3.1 (template change -> re-copy + cv flush on
prod). PRODUCTION_CUTOVER.md change log updated.

civi-extension/webform-mw/info.xml

@@ -17,7 +17,7 @@
     <url desc="Main Extension Page">https://github.com/joelbrock/WebForm-mw</url>
   </urls>
   <releaseDate>2026-06-05</releaseDate>
-  <version>0.3.0</version>
+  <version>0.3.1</version>
   <develStage>beta</develStage>
   <compatibility>
     <ver>5.50</ver>