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

package.json

@@ -5,6 +5,7 @@
   "scripts": {
     "dev": "next dev",
     "build": "next build",
+    "export": "STATIC_EXPORT=true next build",
     "start": "next start",
     "lint": "biome lint --write .",
     "format": "biome format --write .",