Add the Layout tab: five areas with modules, columns and a menu editor

A blog page is now header, left column, main content, right column and
footer, each holding an ordered list of modules (blog title, logo, menu,
archive by year/month, recent posts, custom HTML, footer text, RSS link,
site map). Side columns have percentage widths, can be hidden, and can
keep their space when empty; on phones they stack under the posts. The
menu becomes its own ordered list mixing pages and outside links.
Announcements are placed per column at its top or bottom.

Modules and menu items get tables; the migration derives them from each
blog's old theme (nav position, footer text, show-in-nav pages) so
existing blogs look the same. Custom HTML is deliberately stored and
served as-is (owner's decision; see AGENTS.md for why the blast radius
is the blogger's own origin).

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

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