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/dashboard/kpi-cards.tsx

@@ -28,8 +28,8 @@ function formatPercent(value: number): string {
 
 function KpiTileSkeleton() {
   return (
-    <div className="relative overflow-hidden rounded-xl border border-slate-200 bg-card p-5 shadow-sm">
-      <div className="absolute inset-x-0 top-0 h-1 bg-slate-100" aria-hidden />
+    <div className="relative overflow-hidden rounded-xl border border-border bg-card p-5 shadow-sm">
+      <div className="absolute inset-x-0 top-0 h-1 bg-muted" aria-hidden />
       <Skeleton className="h-3 w-24" />
       <Skeleton className="mt-3 h-7 w-32" />
       <Skeleton className="mt-2 h-3 w-12" />