Reduce duplication in layouts and CSS #229

Problem

Several duplication smells:

1. .about-photo and .blog-photo are identical except object-position.
2. _layouts/post.html re-declares .page-header styling that main.css already has.
3. Every post hand-duplicates its front-matter title in a body <h1 class="page-title">, inviting the two to drift apart.
4. The head repeats the same excerpt/description fallback chain three times where a single {% capture %} would do.

Proposal

Merge the photo classes, drop the redundant inline styles, render the post <h1> (and header image) from front matter in the post layout, and capture the meta description once. Specs locating by class or asserting titles must follow.

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