An item's text takes four fifths of the reading pane, not 80 characters (#360)

Asked for as a share of the window. The pane is the window less the
feed list, so a share of the window would never bind; a share of the
pane does. On a phone the pane is the screen, and keeps all of it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/CHANGELOG.md

@@ -13,7 +13,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 
-- An item's title and text keep to about 80 characters a line, however wide the window, so the eye finds the start of the next line (#360).
+- An item's title and text take four fifths of the reading pane's width, no longer running to its far edge (#360).
 
 ### Added