fix(issues): sync sticky comment header background with highlight fade (MUL-3759)

The deep-link highlight tint faded out over 700ms on the comment body
layers but the sticky header's background switched instantly, and its
4px bottom `after` gradient band recolored by class-switching that
`transition-colors` cannot animate. Both desynced from the body during
the fade, showing a white header and a pale seam under it.

Add `transition-colors duration-700` to the sticky shell so the header
background fades with the body, and make the `after` band derive its
color from the header via `bg-[inherit]` + a `mask-image` fade instead
of a per-state gradient color, so all three layers are driven by the
single header background transition.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/views/issues/components/comment-card.tsx

@@ -46,8 +46,6 @@ import { deriveThreadResolution } from "./thread-utils";
 
 const highlightedCommentBackgroundClass =
   "bg-[color-mix(in_srgb,var(--card)_95%,var(--brand)_5%)]";
-const highlightedCommentFadeClass =
-  "after:from-[color-mix(in_srgb,var(--card)_95%,var(--brand)_5%)]";
 
 function StickyHeaderShell({
   className,
@@ -67,9 +65,8 @@ function StickyHeaderShell({
   return (
     <div
       className={cn(
-        "sticky top-0 z-10 after:pointer-events-none after:absolute after:inset-x-0 after:top-full after:h-1 after:bg-gradient-to-b after:to-transparent",
+        "sticky top-0 z-10 transition-colors duration-700 after:pointer-events-none after:absolute after:inset-x-0 after:top-full after:h-1 after:bg-[inherit] after:[mask-image:linear-gradient(to_bottom,#000,transparent)] after:[-webkit-mask-image:linear-gradient(to_bottom,#000,transparent)]",
         highlighted ? highlightedCommentBackgroundClass : "bg-card",
-        highlighted ? highlightedCommentFadeClass : "after:from-card",
       )}
     >
       <div className={className}>