[codex] Fix legacy post table of contents

[YoonKeumJae]

Summary

• Prepares post HTML before render by sanitizing and injecting heading IDs into the HTML string.
• Reuses the prepared HTML for both post rendering and table-of-contents extraction.
• Keeps the existing post-render enhancement path for media, links, and code blocks.

Root cause

Legacy post content had valid H1 elements from the API, but those headings could remain without IDs in the final rendered DOM. The TOC only includes headings with IDs, so affected pages showed only the post title.

Validation

• Deployed this branch to Azure SWA test
• Verified the OneNote calendar page shows the post title plus the three H1 entries in the TOC.
• npm run lint
• npm run build

Note: build still emits existing Vite dependency export and chunk-size warnings, but exits 0.