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]>

docs/PRD.md

@@ -28,6 +28,11 @@ A single-page web app that answers three questions:
 ## Users
 
 One persona: a player of 2–5 gacha games who checks in a few times a week, most often on mobile.
+
+Most often is not only, and the desktop layout is not the phone layout stretched. Past `lg` the page
+splits: what it is *telling* the reader to do — the next deadlines, tonight's dailies — pins to a
+rail on the left and stays put, while the lists it is *showing* them scroll beside it. Below that
+breakpoint the same split produces the same answer in one column: instructions first.
 They care about accuracy of end dates above everything else — a wrong date is worse than a missing
 event, because a missing event sends them to a wiki while a wrong one makes them miss content.