feat: URL-driven catchall routing, drop sidebar nav, split export build

- app/[[...slug]]/page.tsx replaces app/page.tsx; activeSlug from URL params
- SidebarNav and MobileNav removed from main layout (sections accordion is the nav)
- next.config.ts: output:export controlled by STATIC_EXPORT env var instead of NODE_ENV
- package.json: yarn build is standard Next.js build; yarn export is STATIC_EXPORT=true
- Mock API route: force-static + generateStaticParams for output:export compatibility

app/[[...slug]]/page.tsx

@@ -0,0 +1,50 @@
+import { notFound } from 'next/navigation';
+import type { SectionRecord } from '$entities/Section';
+import { getCollection } from '$shared/api';
+import { SectionFactory } from '$widgets/SectionFactory';
+import { SectionsAccordion } from '$widgets/SectionsAccordion';
+
+/**
+ * Optional catchall: `/` → first section, `/:slug` → that section.
+ */
+export async function generateStaticParams() {
+  const { items: sections } = await getCollection<SectionRecord>('sections', {
+    sort: 'order',
+  });
+  return [{}, ...sections.map((s) => ({ slug: [s.slug] }))];
+}
+
+type Props = {
+  params: Promise<{ slug?: string[] }>;
+};
+
+/**
+ * Portfolio page — one route per section, sections list always visible.
+ */
+export default async function SectionPage({ params }: Props) {
+  const { slug } = await params;
+
+  const { items: sections } = await getCollection<SectionRecord>('sections', {
+    sort: 'order',
+  });
+
+  if (sections.length === 0) {
+    notFound();
+  }
+
+  const activeSlug = slug?.[0] ?? sections[0].slug;
+
+  if (!sections.some((s) => s.slug === activeSlug)) {
+    notFound();
+  }
+
+  return (
+    <main className="px-8 py-12 lg:py-16 lg:px-16">
+      <SectionsAccordion sections={sections} activeSlug={activeSlug}>
+        {sections.map((s) => (
+          <SectionFactory key={s.slug} slug={s.slug} />
+        ))}
+      </SectionsAccordion>
+    </main>
+  );
+}

app/api/collections/[collection]/records/route.ts

@@ -1,6 +1,6 @@
 import { NextResponse } from 'next/server';
 
-export const dynamic = 'force-dynamic';
+export const dynamic = 'force-static';
 
 const base = { created: '', updated: '' };
 
@@ -241,6 +241,10 @@ const FIXTURES: Record<string, unknown[]> = {
   ],
 };
 
+export function generateStaticParams() {
+  return Object.keys(FIXTURES).map((collection) => ({ collection }));
+}
+
 /**
  * Mock API route handler for PocketBase collection records.
  * Returns fixture data shaped as a PocketBase list response.