feat: smooth view transitions, no skeleton flash

User pain point: skeleton loading.tsx files made the app feel like
a sequence of page reloads, exposing backend latency. Replaced with
a single 1px shimmer bar + crossfade via React's <ViewTransition>.

Changes:
- Enable experimental.viewTransition in next.config.ts
- Add SmoothViewTransition wrapper (ViewTransition name=page-content)
- Add LoadingFade component: thin animated bar instead of skeleton
- Add RouteAnnouncer for a11y (screen readers + focus reset)
- Add ::view-transition-old/new CSS for the crossfade (220ms, no
  jarring slide, respects prefers-reduced-motion)
- Wrap admin/tuxedo/IRD layout children in SmoothViewTransition
  (sidebar/header/footer stay mounted; only the body fades)
- Replace 19 skeleton loading.tsx files with the fade component

Result: navigation now feels like a single app, not a series of
preload-and-render events. The user never sees a 'skeleton of the
page they're about to load.'

src/app/checkout/loading.tsx

@@ -1,65 +1,6 @@
-import type { Metadata } from "next";
-
-export const metadata: Metadata = {
-  title: "Processing...",
-  description: "Loading checkout...",
-  robots: {
-    index: false,
-    follow: false,
-  },
-};
+import { LoadingFade } from "@/components/transitions/LoadingFade";
 
+/** Subtle navigation indicator — see LoadingFade for rationale. */
 export default function Loading() {
-  return (
-    <div className="min-h-screen bg-gradient-to-br from-stone-50 via-white to-emerald-50/30">
-      <div className="h-20 border-b border-slate-100/50" />
-      
-      <main className="px-6 py-12">
-        <div className="mx-auto grid max-w-6xl gap-8 md:grid-cols-[1fr_400px]">
-          <div>
-            {/* Title skeleton */}
-            <div className="h-10 w-32 rounded-lg bg-slate-200 animate-pulse" />
-            <div className="h-5 w-64 rounded mt-3 bg-slate-100 animate-pulse" />
-            
-            {/* Form skeleton */}
-            <div className="mt-8 space-y-6">
-              <div className="rounded-2xl bg-white p-6 shadow-sm border border-slate-100">
-                <div className="h-6 w-40 rounded bg-slate-100 animate-pulse" />
-                <div className="mt-4 space-y-4">
-                  <div className="space-y-2">
-                    <div className="h-4 w-20 rounded bg-slate-100 animate-pulse" />
-                    <div className="h-12 w-full rounded-xl bg-slate-100 animate-pulse" />
-                  </div>
-                  <div className="space-y-2">
-                    <div className="h-4 w-16 rounded bg-slate-100 animate-pulse" />
-                    <div className="h-12 w-full rounded-xl bg-slate-100 animate-pulse" />
-                  </div>
-                </div>
-              </div>
-              
-              {/* Button skeleton */}
-              <div className="h-14 w-full rounded-xl bg-slate-100 animate-pulse" />
-            </div>
-          </div>
-          
-          {/* Sidebar skeleton */}
-          <aside>
-            <div className="rounded-2xl bg-white p-6 shadow-sm border border-slate-100 sticky top-6">
-              <div className="h-6 w-32 rounded bg-slate-100 animate-pulse" />
-              <div className="mt-4 space-y-3">
-                {[1, 2, 3].map((i) => (
-                  <div key={i} className="flex justify-between">
-                    <div className="h-4 w-32 rounded bg-slate-100 animate-pulse" />
-                    <div className="h-4 w-16 rounded bg-slate-100 animate-pulse" />
-                  </div>
-                ))}
-              </div>
-            </div>
-          </aside>
-        </div>
-      </main>
-      
-      <span role="status" className="sr-only">Loading checkout...</span>
-    </div>
-  );
-}
+  return <LoadingFade />;
+}