feat: add mobile-responsive layout and card views across all pages

- New useIsMobile hook and MobileCardList component
- AppLayout: hamburger menu + Drawer navigation on mobile
- All list pages (Inbox, Posteingang, Manuell, Mail, Freigabe, Zahlung, TaskLog)
  show card layout on mobile instead of tables
- CSS: responsive modal height, horizontal table scroll, text-size-adjust
- Backend: Agrarmonitor polling fixes, Zahlung service improvements,
  IMAP folder service extended, misc controller fixes

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

paperless-frontend/src/hooks/useIsMobile.ts

@@ -0,0 +1,11 @@
+import { Grid } from 'antd';
+
+/**
+ * Zentraler Breakpoint-Hook: mobil = Viewport unter `md` (768px).
+ * Beim allerersten Render liefert useBreakpoint() noch keine Werte —
+ * dann Desktop annehmen, um ein Layout-Flackern zu vermeiden.
+ */
+export function useIsMobile(): boolean {
+  const screens = Grid.useBreakpoint();
+  return screens.md === undefined ? false : !screens.md;
+}