A reload offer only for a page that is older, and a tab switch that moves one scroller

The update toast fired whenever a service worker was waiting, so after every
release it appeared on pages that were already the release -- including one
fetched with Ctrl+Shift+R. It now compares the waiting worker's release with
the page's own, and so does the reload that follows another tab accepting it.

On Admin -> Prompts, visually hidden labels were positioned against the page
and made the document 6771px tall behind an overflow-hidden root; the tab
handler's scrollIntoView then scrolled that root and lifted the shell 56px.
Every scroll region is now a containing block, and the handler moves only the
container that scrolls.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/lembas/web/static/css/app.css

@@ -452,7 +452,17 @@ input.visually-hidden[type="checkbox"] {
    child will not shrink below its content without it, so a scroller missing it
    grows its parent instead of scrolling inside it. `.thread-scroll` relied on a
    scroll container's automatic minimum size to get away with omitting it, which
-   is true and is not something the next person should have to know. */
+   is true and is not something the next person should have to know.
+
+   `position: relative` makes each scroller the containing block for what is in
+   it, and without it a `.visually-hidden` label is not in it at all. That class
+   is `position: absolute`, so with no positioned ancestor it is placed against
+   the *page*, at its static position -- six thousand pixels down the Tools
+   panel on /admin/prompts -- and the document grew to 6771px behind a root
+   that is `overflow: hidden`. Nobody can scroll that by hand, but
+   `scrollIntoView()` and `focus()` scroll every ancestor that can scroll,
+   and the root can. Switching a tab there lifted the whole shell 56px: the
+   topbar gone off the top and a strip of bare background under everything. */
 .scroll-region,
 .sidebar__scroll,
 .inspector__body,
@@ -460,6 +470,7 @@ input.visually-hidden[type="checkbox"] {
 .thread-scroll,
 .admin-scroll,
 .main > .tabs > .tabs__body {
+  position: relative;
   flex: 1;
   min-height: 0;
   overflow-y: auto;