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/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.