Add a page width: cap the columns at so many pixels, centred or at the left

The header's and footer's text follows the cap when they already follow
the main column; their backgrounds stay full width. The inset that lines
them up with the main column is now exact — the wrap's 1em plus the
column's share of the wrap's content box — and, with a page width, uses
min() so it is right whether the page or the screen is the narrower.

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

1 files changed, 10 insertions, 5 deletions

diff --git a/AGENTS.md b/AGENTS.md
index 4cf36a3..8459d6d 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -240,11 +240,16 @@ internal/web/         server.go (host router, middleware, render helpers)
   laid out only when it has modules, or always with `theme.keep_columns`
   (`HasLeft/HasRight` → `body.has-*`
   classes; column widths in the theme CSS under `@media (min-width: 701px)`).
-  The page is always full width — there is no content max-width; the
-  column percentages divide the page and `keep_columns` is how a blogger
-  gets a narrower, centred main column, with `align_to_main` insetting the
-  header's and footer's `.wrap`s by the column widths (`calc(1em + N%)`,
-  wide screens only, backgrounds untouched) so they line up with it;
+  The page is the screen's width unless `page_width` (pixels, 0 = screen,
+  clamped 400–3000) caps `.wrap.body-wrap` with `max-width`, centred or at
+  the left per `page_align` (`Theme.PageMargin`); the column percentages
+  divide the page either way and `keep_columns` is how a blogger gets a
+  narrower main column. `align_to_main` insets the header's and footer's
+  `.wrap`s to the main column's edges (`Theme.Inset`: the wrap's 1em plus
+  the column's share of the wrap's content box, `calc((1-2w/100)em + w%)`;
+  with a page width those wraps get the same `max-width`/margin and the
+  share is `min(w%, w% of the page width)`, the px value alone first for
+  browsers without `min()` — wide screens only, backgrounds untouched);
   below 700px `.cols` becomes block flow so the order is main → left →
   right → footer. Everything is rendered by the `module` template in
   `layouts/blog.html`. New blogs get `insertDefaultModules` (title + menu