fix(ui): give the scroll shadow size variable a real default

The base rule declared --scroll-shadow-size as var(--scroll-shadow-size,
48px) — a self-referential cycle, which computes to invalid and silently
resolves every mask below to none. The old ScrollShadow component masked
the bug by setting the variable inline; the hook-based path (the chat
scroller) sets only the data attributes, so the chat lost its top and
bottom scroll fades. A concrete 48px default restores them; inline
overrides from the component still win.

packages/ui/src/index.css

@@ -258,7 +258,12 @@ div[data-chat-input-footer="true"] {
 
 /* Scroll shadow (HeroUI) fallback styling to ensure visible gradients without the Tailwind plugin */
 [data-scroll-shadow="true"] {
-  --scroll-shadow-size: var(--scroll-shadow-size, 48px);
+  /* A concrete default, not var(--scroll-shadow-size, 48px): a custom
+     property referencing itself is a cycle and computes to invalid, which
+     silently killed every mask below for consumers that don't set the
+     variable inline (the hook-based chat scroller). Inline styles from the
+     ScrollShadow component still override this. */
+  --scroll-shadow-size: 48px;
 }
 
 [data-scroll-shadow="true"][data-orientation="vertical"] {