fix(audit-wave-9): route-level loading skeletons across dashboard

Add a default [portSlug]/loading.tsx that covers all 72 nested routes
that previously rendered nothing during the cold-load gap. Uses the
existing PageSkeleton (page-header + table-skeleton) so the empty-header
flash on direct-URL visits / tab navigations is gone.

Add tailored loading.tsx for the four other tab-strip detail surfaces so
their initial paint mirrors the real page structure (header strip,
pipeline stepper for interests, tab strip, two-column overview):

- yachts/[yachtId]/loading.tsx
- companies/[companyId]/loading.tsx
- interests/[interestId]/loading.tsx
- berths/[berthId]/loading.tsx

(clients/[clientId]/loading.tsx already existed.)

Closes ui/ux M3.

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

src/app/(dashboard)/[portSlug]/berths/[berthId]/loading.tsx

@@ -0,0 +1,29 @@
+import { Skeleton } from '@/components/ui/skeleton';
+import { CardSkeleton } from '@/components/shared/loading-skeleton';
+
+export default function Loading() {
+  return (
+    <div className="space-y-6">
+      <div className="space-y-3 rounded-xl border border-border bg-card px-5 py-4 shadow-sm">
+        <div className="flex items-center gap-3">
+          <Skeleton className="h-12 w-13 rounded-2xl" />
+          <Skeleton className="h-7 w-32" />
+          <Skeleton className="h-6 w-24 rounded-full" />
+        </div>
+        <div className="flex flex-wrap gap-2">
+          <Skeleton className="h-9 w-32 rounded-md" />
+          <Skeleton className="h-9 w-20 rounded-md" />
+        </div>
+      </div>
+      <div className="flex gap-2 border-b border-border pb-1">
+        {Array.from({ length: 4 }).map((_, i) => (
+          <Skeleton key={i} className="h-8 w-24 rounded-md" />
+        ))}
+      </div>
+      <div className="grid grid-cols-1 gap-6 md:grid-cols-2">
+        <CardSkeleton />
+        <CardSkeleton />
+      </div>
+    </div>
+  );
+}