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/ui/kpi-tile.tsx

@@ -44,7 +44,7 @@ export function KPITile({
   return (
     <div
       className={cn(
-        'group relative overflow-hidden rounded-xl border border-slate-200 bg-gradient-brand-soft p-5 shadow-sm transition-all duration-base ease-smooth hover:shadow-md',
+        'group relative overflow-hidden rounded-xl border border-border bg-gradient-brand-soft p-5 shadow-sm transition-all duration-base ease-smooth hover:shadow-md',
         className,
       )}
       {...props}