#301: the overflow was an escaping visually-hidden heading

Found by standing up the local stack instead of guessing through CI.
The DOM tree under `.app-body` shows it in one line:

  span.visually-hidden rect=[342,343] pos=absolute

Its right edge is 343, and `.app-body` reports scrollWidth 343 against a
320 client. The table's actions column carries a `.visually-hidden`
heading, which is `position: absolute`. `.table-scroll` was `position:
static`, so it was NOT that span's containing block — the span escaped
the scroller's clipping, kept its static position out at the table's
right edge, and pushed the page.

`position: relative` on the wrapper makes it the containing block, and
the span is clipped like the rest of the table.

This is one cause behind both numbers: 23px locally, matching the
original report, and 417px in CI, where different font metrics make the
table wider and carry the span further out. Chasing them as separate
problems is what cost three CI rounds.

Verified locally against a real stack: the reflow guard passes and the
whole a11y pack is green, 11 tests in both colour schemes.

apps/web/src/styles/base.css

@@ -972,6 +972,13 @@ button {
 .table-scroll {
   overflow-x: auto;
   max-width: 100%;
+  /* Establishes the containing block for the table's absolutely positioned
+     descendants — the `.visually-hidden` column headings. Without it their
+     containing block is `.app-body`, so they escape this scroller's clipping,
+     keep their static position out at the table's right edge, and push the
+     PAGE (issue #301). Exactly 23px locally, 417px in CI, where different
+     font metrics make the table wider — one cause, two numbers. */
+  position: relative;
 }
 
 .table {