Add a mobile layout (48rem breakpoint) and the missing viewport meta

Pages now declare width=device-width, so phones render at real device
width instead of a scaled-down 980px layout viewport. Below 48rem the
content becomes a single column with the sidebar lifted above the
article as a wrapping link strip; floated .left/.right figures fall back
to plain centered figures (explicit img widths still shrink-wrap) and
.wide keeps its centered full-viewport bleed.

pagerite/views.py

@@ -128,6 +128,9 @@ def _layout(
     order intact.
     """
     doc = Document(E.Title, lang="en")
+    # Responsive layout (see the 48rem breakpoint in pagerite.css) needs
+    # the real device width, not the default 980px layout viewport.
+    doc.meta(name="viewport", content="width=device-width, initial-scale=1")
     # A custom favicon (from the site editor) is linked explicitly; without
     # one, browsers fall back to the build's /favicon.ico by convention.
     if favicon: