Add post summaries, featured images and a page outro

Recreating gizmolab.net as a blog showed three things the design system
could not express, all of them ordinary blog features:

- Listings can show a summary and a "Read more" link instead of the whole
  post (theme.list_style). The summary is the source up to <!--more-->;
  without the marker a Markdown post is cut at a block boundary after 70
  words (never inside a code fence) and an HTML post is shown whole, since
  cutting hand-written markup blind would leave tags open. Computed at
  render time (Post.Excerpt/HasMore), nothing new is stored.
- A post can carry a featured image (posts.image, a library file that is
  unset when the file is deleted): a thumbnail floated beside the entry on
  listings, and on the post page above or below the text, or not at all,
  as theme.post_image says. The picker is the design page's imagepick; its
  library panel moves out of design.html into partials/imagelib.html so
  both forms share it — which also removes a stale pickAction call the
  design form's click handler would have hit.
- A page can have a closing text after its posts (pages.outro_md/html), in
  the intro's format: the per-page counterpart of the blog-wide "after the
  posts" module, for a home page that ends in a site map or a widget. The
  editor partial takes formatof to share another editor's Format row, and
  its upload field is now <name>_file so two editors fit in one form.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Sd8UPWrvyYCLj97JexNw3A

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