Let an application replace the safe-area insets and dock on the bottom bar

Components read the device's safe-area insets straight from env(), which a
browser test cannot fake and an application drawing its own status strip
cannot extend. Every inset the package reads, in its CSS and its views, is
now var(--material-safe-top|bottom|left|right, env(safe-area-inset-…)):
unchanged while the variables are unset.

The app shell's --material-bottom-bar also adds
var(--material-bottom-extra, 0px), so an application that docks something on
the phone's navigation bar (an offline banner) sets its height once and the
snackbar, a fab button and the page's bottom padding clear it.

A guard test fails on any env(safe-area-inset-*) outside such a variable, and
AppShellTest's assertion on the bar height's class follows the new value.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RHoXZSHc8gGpZjFmA5fPc2

resources/views/showcase/shell.blade.php

@@ -59,7 +59,7 @@
             </x-slot:actions>
 
             <x-slot:top>
-                <header class="sticky top-0 z-20 flex h-16 items-center gap-1 bg-surface px-1 pt-[env(safe-area-inset-top)] sm:px-4">
+                <header class="sticky top-0 z-20 flex h-16 items-center gap-1 bg-surface px-1 pt-[var(--material-safe-top,env(safe-area-inset-top))] sm:px-4">
                     <span class="sm:hidden"><x-livewire-material::button icon="menu" tooltip="Open navigation" x-data x-on:click="$store.rail.show()" data-test="shell-menu" /></span>
                     <h1 class="min-w-0 flex-1 truncate px-3 type-title-lg sm:px-0">{{ $current['title'] }}</h1>
                     <x-livewire-material::button icon="search" tooltip="Search" />