Add pinned posts

Branch feature/pinned-posts · version 0.5.0

What changed

• New pinned key in a post's header block: pinned: true keeps the post at the top of the index page. It's optional and defaults to false.
• Several pinned posts are listed first, newest first, followed by every other post in date order.
• "Pinned" label: pinned posts show Pinned before the date on the index page, so a pinned post with an older date doesn't look out of order.
• "Why This Project Exists" is pinned.
• Docs: the hello-world guide, the About page and the README list pinned as the sixth header key.
• Version: 0.5.0.

Notes

• Pinning only affects the index page. The RSS feed, the newer/older links on each post and the About page's "Latest post" stay in date order, so pinning a post doesn't reshuffle the reading chain.
• An invalid value such as pinned: yes skips the post and logs the error, the same as an invalid draft value. Accepted values are true and false (also 1/0, t/f).

Testing

• All tests pass with the race detector (go test -race ./...).
• New test: pinned posts first and newest first among themselves, the rest by date, two "Pinned" labels, an invalid value skipped, and the newer/older links and feed unaffected.
• Built and ran locally with the real posts: "Why This Project Exists" is first with the Pinned label, and its "older" link still goes to the Go 1.27 post.

Screenshots

From the branch's build running locally.

Desktop	Mobile

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