Let the rail centre its destinations

Plan step 25 (navigation Missing): M3's "alignment: top or center; on tablets
prefer center alignment for reach" had no implementation — the rail was
top-aligned only. `<x-navigation-rail align="center">` centres the destinations
between the header and the footer, which stay at their ends because M3 keeps the
menu icon and the FAB top-aligned. `safe center` gives the top back the moment
there are more destinations than fit, where centring would push the first one
out of reach above the scroller.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Qwx5USif3wFFmxtHg5U1g9

resources/views/showcase/sections/navigation.blade.php

@@ -58,7 +58,7 @@
                 </div>
             </div>
             BLADE,
-        'Collapsed, narrow and expanded rails' => <<<'BLADE'
+        'Collapsed, narrow (centred) and expanded rails' => <<<'BLADE'
             <div class="flex w-full flex-wrap items-start gap-6">
                 <div class="flex h-[36rem] overflow-hidden rounded-corner-lg border border-outline-variant">
                     <x-navigation-rail mode="collapsed" label="Collapsed example" divider>
@@ -81,7 +81,7 @@
                 </div>
 
                 <div class="flex h-[36rem] overflow-hidden rounded-corner-lg border border-outline-variant">
-                    <x-navigation-rail mode="collapsed" width="narrow" label="Narrow example" divider>
+                    <x-navigation-rail mode="collapsed" width="narrow" align="center" label="Narrow example" divider>
                         <x-slot:header>
                             <x-fab label="Compose" icon="edit" />
                         </x-slot:header>