Task #450: switch html/body horizontal clip from hidden to clip

After Task #449 the sticky Notes header was still scrolling away.
Root cause: `overflow-x: hidden` on `html, body` makes both
elements scroll containers. Body sized itself to its content
(no internal scroll happens), so a `position: sticky` descendant
got scoped to a non-scrolling container and rode off-screen with
the page.

`overflow-x: clip` clips overflowing content the same visual way
`hidden` does, but it does NOT establish a scroll container, so
sticky descendants continue to resolve against the viewport and
pin correctly.

Changed only `artifacts/tx-os/src/index.css` (the `html, body`
rule added in #449). All other rules untouched. `overflow: clip`
is supported in Chrome 90+, Firefox 81+, Safari 16+.

artifacts/tx-os/src/index.css

@@ -116,7 +116,7 @@
   }
   html,
   body {
-    overflow-x: hidden;
+    overflow-x: clip;
   }
 }