Fix Hekouwang theme thumbnail

[cursor]

Summary

• Move the Hekouwang post into _posts/theme/ so it appears in the theme gallery.
• Change its front matter to layout: theme, category: theme, and thumbnail: hekouwang.png.
• Move hekouwang.png into media/thumbnails/, the path used by gallery cards.

Verification

• Waited for the GitHub Pages deployment for latest gh-pages to complete, then waited 10 minutes before checking the live site.
• Confirmed live https://theme.typora.io/ did not list Hekouwang and https://theme.typora.io/media/thumbnails/hekouwang.png returned 404, while the misplaced https://theme.typora.io/thumbnails/hekouwang.png returned 200.
• Ran bundle exec jekyll build locally after installing Ruby/Bundler in the environment.
• Confirmed generated _site/index.html includes Hekouwang and /media/thumbnails/hekouwang.png, generated _site/theme/Hekouwang/index.html exists, and generated _site/media/thumbnails/hekouwang.png exists.

Post-Deploy Monitoring & Validation

• After merge/deploy, check https://theme.typora.io/ for the Hekouwang gallery card.
• Confirm https://theme.typora.io/media/thumbnails/hekouwang.png returns 200 image/png.
• Confirm https://theme.typora.io/theme/Hekouwang/ returns 200 and shows the theme page.
• No additional operational monitoring required; this is a static content metadata/resource placement fix.