feat(theme): rail-btn-block full-width rail-footer modifier (0.4.0)

HistorianGateway hand-rolled a .rail-btn override purely to make the rail-foot
sign-out button span the rail; its own comment flagged folding it into the kit.
Adds a composable modifier instead: class="rail-btn rail-btn-block".

Uses display:block + width:auto, NOT width:100%. The hand-rolled overrides set
width:100%, which resolves against the containing block and then adds .rail-btn's
0 0.6rem side margins on top, overflowing the rail by 1.2rem. width:auto fills
the same inset correctly.

Scope note: the app overrides that motivated this do more than full-width (they
also restyle font-size, padding, border, background and hover). Adopting the
modifier removes the width hack; whether to also drop the restyle is each app's
call, since it changes their rail appearance.

Version 0.3.1 -> 0.4.0 (additive). Build 0 errors, 48/48 tests, nupkg verified
to carry the new rule in staticwebassets/css/layout.css. Not yet published.

ZB.MOM.WW.Theme/README.md

@@ -81,6 +81,13 @@ Served at `_content/ZB.MOM.WW.Theme/…` by the ASP.NET static-web-asset pipelin
 |---|---|
 | `css/theme.css` | Design tokens (`--accent`, `--ok`, `--warn`, …), typography, utility helpers |
 | `css/layout.css` | Side-rail shell layout, collapsible nav, `StatusPill` variants, card/field helpers |
+
+Rail-footer actions: `class="rail-btn"` is the inline default; add `rail-btn-block`
+(`class="rail-btn rail-btn-block"`) for a full-width footer button. Use the modifier rather than
+redeclaring `.rail-btn` in an app stylesheet — a local override that sets `width: 100%` overflows
+the rail, because the width resolves against the containing block and the inline version's
+`0 0.6rem` side margins are then added on top.
+
 | `fonts/ibm-plex-sans-400.woff2` | IBM Plex Sans Regular |
 | `fonts/ibm-plex-sans-600.woff2` | IBM Plex Sans SemiBold |
 | `fonts/ibm-plex-mono-500.woff2` | IBM Plex Mono Medium |