fix(theme): rail-btn-block must use an explicit width (0.4.1)

0.4.0's rail-btn-block used display:block + width:auto, which fills for an <a>
but SHRINK-WRAPS a <button> — a form control does not take the fill-available
behaviour a non-replaced block box gets. Measured in one 197px rail-foot
container: <a> filled to 177.42px (0 short), <button> came up 114px short.

That matters because a rail foot normally holds both element types: Sign In is
an anchor, Sign Out is a form-submit button. So 0.4.0's modifier got one right
and one wrong, and the wrong one is what signed-in operators see — worse than
the width:100% overflow it was meant to replace (~10px of visible clipping).

Fixed with an explicit calc(100% - 1.2rem) + box-sizing:border-box, which
measures 0px short for both element types. Both rejected shorthands and their
measured failures are recorded in the CSS so neither is reintroduced as a
'simplification'.

Found by HistorianGateway while evaluating adoption; it held off adopting
rather than trading one defect for a larger one. No consumer had adopted the
modifier, so nothing shipped broken. The .btn half of 0.4.0 is unaffected and
is already live-verified in three apps.

Build 0 errors, 48/48 tests.

ZB.MOM.WW.Theme/Directory.Build.props

@@ -4,7 +4,7 @@
     <Nullable>enable</Nullable>
     <ImplicitUsings>enable</ImplicitUsings>
     <LangVersion>latest</LangVersion>
-    <Version>0.4.0</Version>
+    <Version>0.4.1</Version>
     <ManagePackageVersionsCentrally>true</ManagePackageVersionsCentrally>
   </PropertyGroup>
 </Project>