fix: use getTagSlug() for tag links to prevent empty tag pages

Tags containing both spaces and dashes (e.g. "Writings - 創作") produced
mismatched slugs: inline generation created "writings---創作" while
getTagSlug() collapsed dashes to "writings-創作", causing tag pages to
show no articles.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/sitemap.ts

@@ -1,5 +1,6 @@
 import { MetadataRoute } from 'next';
 import { allPosts, allPages } from 'contentlayer2/generated';
+import { getTagSlug } from '@/lib/posts';
 
 export default function sitemap(): MetadataRoute.Sitemap {
   const siteUrl = process.env.NEXT_PUBLIC_SITE_URL || 'http://localhost:3000';
@@ -58,7 +59,7 @@ export default function sitemap(): MetadataRoute.Sitemap {
   );
 
   const tagPages = allTags.map((tag) => ({
-    url: `${siteUrl}/tags/${encodeURIComponent(tag.toLowerCase().replace(/\s+/g, '-'))}`,
+    url: `${siteUrl}/tags/${encodeURIComponent(getTagSlug(tag))}`,
     lastModified: new Date(),
     changeFrequency: 'weekly' as const,
     priority: 0.5,