feat: improve frontend accessibility, dark mode, and UX

- Replace div with semantic button elements in AppLayout for accessibility
- Use Ant Design theme tokens instead of hardcoded colors in MailDetailPage
- Add loading state to refresh buttons and pagination info
- Add German empty state messages to tables
- Clean up unnecessary ConfigProvider wrappers

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

paperless-frontend/src/pages/InboxDetailPage.tsx

@@ -1176,7 +1176,7 @@ export default function InboxDetailPage() {
           <Button icon={<ArrowLeftOutlined />} onClick={() => navigate('/inbox')}>
             Zurück
           </Button>
-          <Title level={4} style={{ margin: 0 }}>
+          <Title level={3} style={{ margin: 0 }}>
             {file.name}
           </Title>
           <SourceTag source={file.source} />