Add M3 error pages and the Markdown mail theme

Error pages for 401 to 503 in an error-view root appended to view.paths,
with an inline fallback when the Vite build is missing, and a mail theme
rendered from the application's scheme JSON, with opt-in header and
message components. Completes Phase 9.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01V9NnLxnPp8vaaurb3Z1MFy

tests/Browser/ErrorPagesTest.php

@@ -0,0 +1,54 @@
+<?php
+
+/**
+ * The 404 page, reached the way a visitor reaches it: an address nothing answers.
+ */
+function missingPage(string $colorScheme = 'light')
+{
+    $page = visit('/nothing-lives-here');
+    $page = $colorScheme === 'dark' ? $page->inDarkMode() : $page->inLightMode();
+
+    return $page->waitForEvent('networkidle')
+        ->assertScript("document.readyState === 'complete' && document.querySelector('[data-error-page]') !== null");
+}
+
+/**
+ * Whether the element's computed background is the colour a role resolves to on this page.
+ */
+function paintedIn(string $selector, string $role): string
+{
+    return <<<JS
+        (() => {
+            const probe = document.createElement('div');
+            probe.style.backgroundColor = 'var(--md-sys-color-{$role})';
+            document.body.append(probe);
+            const expected = getComputedStyle(probe).backgroundColor;
+            probe.remove();
+
+            return expected !== 'rgba(0, 0, 0, 0)' && getComputedStyle(document.querySelector('{$selector}')).backgroundColor === expected;
+        })()
+        JS;
+}
+
+it('draws the 404 page in the scheme, in light and dark', function () {
+    missingPage('light')
+        ->assertSee('Page not found')
+        ->assertVisible('[data-error-headline]')
+        ->assertScript("document.documentElement.getAttribute('data-theme') === 'light'")
+        ->assertScript(paintedIn('body', 'surface'))
+        ->assertScript("getComputedStyle(document.querySelector('[data-error-shape] svg')).color !== getComputedStyle(document.body).color")
+        ->assertNoJavaScriptErrors();
+
+    missingPage('dark')
+        ->assertScript("document.documentElement.getAttribute('data-theme') === 'dark'")
+        ->assertScript(paintedIn('body', 'surface'))
+        ->assertScript("getComputedStyle(document.body).backgroundColor !== 'rgb(253, 247, 254)'");
+});
+
+it('fits a 400px screen without scrolling sideways', function () {
+    missingPage()
+        ->resize(400, 800)
+        ->assertVisible('[data-error-headline]')
+        ->assertScript('document.documentElement.scrollWidth <= window.innerWidth')
+        ->assertScript("document.querySelector('[data-error-actions] a').getBoundingClientRect().right <= 400");
+});