Fix PR review follow-ups

[wauputr4]

Summary

• sanitize rendered README HTML before injecting it into project detail pages
• rewrite repo-relative README links/images to GitHub/raw URLs
• normalize scheme-less project homepage URLs before rendering buttons/links
• resolve relative blog thumbnail frontmatter to raw GitHub URLs during sync

Verification

• rtk npm run build
• ASTRO_BASE=/indopensource.org npm run build

Context: PR #15 was already merged, so these are follow-up fixes for the Codex review findings.

[chatgpt-codex-connector]

💡 Codex Review

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Reviewed commit: 3469b408ee

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[chatgpt-codex-connector]

Preserve repo-root README paths

When a README uses GitHub's supported repo-root-relative syntax such as [docs](/docs/guide.md) or ![logo](/assets/logo.png), resolving it with new URL(value, data.html_url/download_url) treats the leading slash as the domain root, producing https://github.com/docs/guide.md or https://raw.githubusercontent.com/assets/logo.png and dropping the owner/repo/branch. This breaks those README links and images on the project detail page; root-relative paths need to be resolved against the repository root URL rather than the file URL.

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