Hold the mail theme to the standard contrast level

A mail client resolves no attribute, so the JSON's top level — the standard roles —
is what it wears, whatever levels the file carries beside them.
Plan: docs/plans/material-3-alignment.md, step 7 (core C2).

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

tests/Feature/MailThemeTest.php

@@ -86,6 +86,23 @@ function withPackageMailComponents(): void
     app()->forgetInstance(Markdown::class);
 }
 
+it('stays on the standard contrast level, which is the one a mail client can show', function () {
+    File::put($this->temporary.'/material-scheme.json', json_encode([
+        'light' => ['primary' => '#123456', 'on-primary' => '#fefefe', 'surface-container' => '#eeeeee', 'surface-container-lowest' => '#fdfdfd'],
+        'dark' => ['primary' => '#abcdef'],
+        'contrast' => [
+            'standard' => 0,
+            'high' => ['light' => ['primary' => '#000000', 'on-primary' => '#ffffff'], 'dark' => ['primary' => '#ffffff']],
+        ],
+    ]));
+
+    config(['livewire-material.scheme' => $this->temporary.'/material-scheme.json']);
+
+    expect(inlineStyle((new ThemedProbeMail)->render(), 'button-primary'))
+        ->toContain('background-color: #123456')
+        ->not->toContain('background-color: #000000');
+});
+
 it('inlines the application\'s scheme onto the mail', function () {
     File::put($this->temporary.'/material-scheme.json', json_encode([
         'light' => ['primary' => '#123456', 'on-primary' => '#fefefe', 'surface-container' => '#eeeeee', 'surface-container-lowest' => '#fdfdfd'],