feat(layout): use the width instead of a column in the middle of the screen

Every screen got the same 672px column, so a desktop reader read a phone page
with two thirds of the window empty — and the one design criticism the release
thread produced was formed from a screenshot.

Past lg the page splits along the line this codebase already draws: what it
tells the reader to do — the next deadlines, tonight's dailies — pins to a
rail on the left and stays there while the lists it shows them scroll beside
it. The footer and the settings panel become columns rather than a long fall
of small grey text. Below that breakpoint nothing moves: the same split puts
the instructions first in one column.

The rail's rule sits on the panel rather than the column. The panel is short
and the list beside it is long, so a full-height divider would spend most of
its length walling off a gap.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

AGENTS.md

@@ -433,6 +433,11 @@ to an open page). Four things hold it up:
 - Keep old fixtures when a source changes shape — the old one is the regression test proving the
   parser still handles the previous format. Fixtures are pinned and permanent; `snapshots/` is the
   current page and gets overwritten. Do not conflate them.
+- **The page is two columns past `lg`, and the split is the one below.** What the page *tells* the
+  reader to do — the next deadlines, tonight's dailies — pins to a rail on the left and stays put
+  while the lists it *shows* them scroll beside it. Below that breakpoint it is one column in the
+  same order. The rail's rule belongs to the panel, not the column: the panel is short and the list
+  is long, so a full-height divider would spend most of its length walling off a gap.
 - **Truncating a list is not re-sorting it.** Each section shows `LIST_CAP` rows and offers "show
   all N". The rows below the cut keep their place in the order, stay counted in the header, and stay
   on the timeline — so the deadline guarantee two bullets down holds for what is hidden exactly as it