Fix missing thumbnails for new Typora themes

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Summary

• Move Retro Typewriter, Formal Dark Pro, and Formal Light Pro posts into the theme category path.
• Replace the unused preview metadata with thumbnail metadata used by the gallery.
• Add 250x200 PNG thumbnails under media/thumbnails/.

Verification

• Confirmed GitHub Actions completed, then waited 10 minutes after the final gh-pages deploy run.
• Checked https://theme.typora.io/ and confirmed the three new themes were absent and their thumbnail URLs returned 404 before this fix.
• Validated the generated thumbnails are PNG files sized 250x200.
• Validated each post has layout: theme, category: theme, and a matching thumbnail file.

Note: local Jekyll build could not be run because Ruby/Bundler are not installed in this environment.