Give browser tests on CI more time and wait for Alpine in the theme test

CI's WebKit job failed twice on a slow runner: a carousel scroll that had
not settled within five seconds, and a press on the theme switch before
Alpine had revealed it. BROWSER_TIMEOUT raises the assertion timeout, and
the workflow sets it to fifteen seconds.

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

tests/Browser/ThemeTest.php

@@ -19,8 +19,10 @@ it('follows the operating system while the visitor has not chosen', function ()
 });
 
 it('keeps the visitor\'s choice over the operating system', function () {
+    // The theme switch waits for Alpine (x-cloak); a press before then would find nothing to press.
     $page = visit('/material')->inDarkMode()
         ->waitForEvent('networkidle')
+        ->assertScript("document.readyState === 'complete' && typeof window.Alpine !== 'undefined' && typeof window.Livewire !== 'undefined'")
         ->assertScript(theme('data-theme', 'dark'));
 
     $page->click('@theme-light')