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

next.config.ts

@@ -1,10 +1,11 @@
 import type { NextConfig } from 'next';
 
-const isExport = process.env.NODE_ENV === 'production';
+/* output: 'export' is opt-in via STATIC_EXPORT=true.
+ * Set this in CI/deploy — not locally — so the mock API route works
+ * during development and local builds. */
+const isExport = process.env.STATIC_EXPORT === 'true';
 
 const nextConfig: NextConfig = {
-  /* output: 'export' only applies at build time — enabling it in dev mode
-   * breaks route handlers (incompatible with force-dynamic in Next.js 16) */
   ...(isExport ? { output: 'export' } : {}),
   images: { unoptimized: true },
 };