style(design): FINDING-008 — scrollbars and selection belong to the shell

A second scrollbar writer injected a style tag at boot painting a zinc thumb with a
violet hover, and because `scrollbar-color` inherits, that zinc pair reached every
scrollable surface in the product. The injector and its dead `CustomScrollbar`
wrapper are gone; globals.css owns scrollbars again through
`--shell-scroll-thumb` / `-hover` (measured: dark rgb(58,54,47), light
rgb(199,192,180) — the previous values were rgb(63,63,70) on rgb(24,24,27)).

`::selection` was a transparent background with a legacy #F8FAFC text colour; it is
now the shell signal at 30% with the shell text colour, in both themes.