feat(sitemap): implement dynamic sitemap index and entity sitemaps

Convert sitemap.xml to a sitemap index referencing split modules
Add dynamic sitemaps for pokedex, habitats, collections, life, and threads
Fetch entity data from API to populate lastmod and priority

frontend/server/routes/sitemap-threads.xml.ts

@@ -0,0 +1,7 @@
+import { normalizeApiBaseUrl, normalizeSiteUrl, threadsSitemapXml } from '../utils/seo-files';
+
+export default defineEventHandler(async (event) => {
+  const config = useRuntimeConfig(event);
+  setHeader(event, 'Content-Type', 'application/xml; charset=utf-8');
+  return threadsSitemapXml(normalizeSiteUrl(config.public.siteUrl), normalizeApiBaseUrl(config.serverApiBaseUrl));
+});