feat: add book detail view with spine-book-model, httpd/book handler, and title links

- Add spine--format-defs defconst shared between index and book models
- Add spine-book-model with status extraction, format selector, notes, recommendation
- Add templates/book.mustache (Pico CSS, journal-style layout)
- Add httpd/book handler serving the detail page
- Link book titles in index view to /book?id=X
- Add spine-book-model-test.el with 13 tests

templates/index.mustache

@@ -78,7 +78,8 @@
     {{#books}}
     <div class="row{{#selected}} selected{{/selected}}">
       <i class="ti {{format_icon}} glyph" aria-hidden="true"></i>
-      <span class="title">{{title}} <span class="muted">· {{author}}</span></span>
+      <a href="{{book_url}}" class="title" style="text-decoration:none;color:inherit">{{title}}</a>
+      <span class="muted">· {{author}}</span>
       {{#tags_inline}}<span class="tags">{{tags_inline}}</span>{{/tags_inline}}
       {{#rec_via}}<span class="trail">via {{rec_via}}</span>{{/rec_via}}
       {{#rating_display}}<span class="trail">{{rating_display}}</span>{{/rating_display}}