fix(ui): polish mobile layouts, error pages, and volume backups

Keep error pages full-viewport, lift the unsaved bar above the
mobile keyboard, make status/heading layouts stack cleanly on small
screens, route service DB backups to volume-backups, and tighten
volume backup grid/forms. Cover with feature tests.

tests/Feature/ErrorPagesRedesignTest.php

@@ -16,6 +16,7 @@ it('renders redesigned error pages with shared shell and actions', function (str
     $html = view($view, ['exception' => $exception])->render();
 
     expect($html)
+        ->toContain('error-page-body')
         ->toContain('error-shell')
         ->toContain('error-code')
         ->toContain('error-title')