feat(migration): complete WordPress migration and UX refinements

- Migrate historical posts, localized assets, and 103 comments
- Colocate media into posts/<slug>/images/ and remove wp-content
- Add native bilingual support for ai-can-change post (English default, /zh variant)
- Support clean taxonomy routing (/category/:category and /tag/:tag)
- Dynamically populate categories and tags in sitemap.xml
- Fix header scroll padding/margin offsets for anchor and reference links
- Constrain image and media widths to prevent content overflow
- Redesign homepage recent posts to 4 compact editorial cards
- Restore valid binary favicon.ico and high-res icon assets
- Remove placeholder content and sample test comments

server/services/content.js

@@ -145,6 +145,7 @@ class ContentService {
         defaultPost.availableLanguages = availableLangs;
         for (const v of Object.values(variants)) {
           v.availableLanguages = availableLangs;
+          v.defaultLang = defaultPost.lang;
         }
 
         this.postsBySlug.set(slug, {