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]/loading.tsx

@@ -0,0 +1,17 @@
+import { PageSkeleton } from '@/components/shared/loading-skeleton';
+
+/**
+ * Default route-level loading UI for every page under `(dashboard)/[portSlug]/...`.
+ *
+ * Renders while the server component resolves the session, port config,
+ * and the client component bootstraps its initial query. Replaces the
+ * empty-header flash on cold direct-URL visits and tab navigations.
+ *
+ * Individual routes can still ship their own `loading.tsx` for a more
+ * tailored skeleton (see `clients/[clientId]/loading.tsx` which mirrors
+ * the detail page's tab strip). When that file exists Next.js uses it
+ * in place of this default.
+ */
+export default function Loading() {
+  return <PageSkeleton />;
+}