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/InboxPage.tsx

@@ -387,7 +387,7 @@ export default function InboxPage() {
             onChange={(e) => setSearch(e.target.value)}
             allowClear
           />
-          <Button icon={<ReloadOutlined />} onClick={load}>
+          <Button icon={<ReloadOutlined />} onClick={load} loading={loading}>
             Aktualisieren
           </Button>
           <Popconfirm