fix(visual): dark-mode-safe borders + sidebar relative + ring-background

Code-review follow-up to PR10b-e:
- DetailHeaderStrip + KPITile: border-slate-200 → border-border so dark
  mode doesn't paint a bright halo around the gradient strip.
- Topbar avatar: ring-white → ring-background so the 2px ring tracks
  the surface (matches the sidebar footer pattern).
- KpiTileSkeleton stripe: bg-slate-100 → bg-muted for parity with
  shadcn skeleton tokens in dark mode.
- Sidebar <aside>: add `relative` so the absolute-positioned
  collapse-toggle button anchors to the sidebar itself rather than
  the nearest positioned ancestor.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/layout/topbar.tsx

@@ -96,7 +96,7 @@ export function Topbar({ ports, user }: TopbarProps) {
         <DropdownMenu>
           <DropdownMenuTrigger asChild>
             <Button variant="ghost" size="icon" className="rounded-full">
-              <Avatar className="w-7 h-7 shadow-sm ring-2 ring-white">
+              <Avatar className="w-7 h-7 shadow-sm ring-2 ring-background">
                 <AvatarImage src={undefined} />
                 <AvatarFallback className="bg-brand text-white text-xs font-semibold">
                   {(user?.name ?? 'U').slice(0, 1).toUpperCase()}