feat(crm): cash-basis revenue + backdatable payment date on mark-paid

Per decision: keep dashboard revenue windows on pure cash basis (recognise
by paid_at for ALL invoices) and give the admin control over paid_at.

- adminDashboard: revert the imported-vs-native split; winSum is paid_at >=
  cutoff for every paid invoice again (clean cash basis).
- BillDetailPage mark-paid dialog: add an optional 'Payment date' field
  (LocalizedDateInput, defaults to today) so a payment can be backdated to
  when it actually arrived. Backend already accepted paidAt end-to-end
  (route validator + markPaid service + payment-log) — only the UI was
  missing. EN/DE 'bills.payment.date' added.

This fixes the collapsed 30=90=365 windows (they were collapsing because
many invoices were marked paid in one session, all stamped 'now').

frontend/src/i18n/locales/de.json

@@ -3818,6 +3818,7 @@
     "payment": {
       "paidAt": "Bezahlt am",
       "amount": "Betrag",
+      "date": "Zahlungsdatum",
       "method": "Methode",
       "reference": "Referenz",
       "notes": "Notizen",

frontend/src/i18n/locales/en.json

@@ -3807,6 +3807,7 @@
     "payment": {
       "paidAt": "Date",
       "amount": "Amount",
+      "date": "Payment date",
       "method": "Method",
       "reference": "Reference",
       "notes": "Notes",

frontend/src/pages/admin/bills/BillDetailPage.tsx

@@ -8,7 +8,7 @@ import { useTranslation } from 'react-i18next';
 import { Link, useNavigate, useParams } from 'react-router-dom';
 import { useQuery, useQueryClient } from '@tanstack/react-query';
 import { ArrowLeft, Eye, Send, CheckCircle, BellRing, XCircle, Truck, Edit2, RefreshCw } from 'lucide-react';
-import { Button, Card, Loading, Input } from '../../../components/common';
+import { Button, Card, Loading, Input, LocalizedDateInput } from '../../../components/common';
 import { DocumentLineageCard } from '../../../components/admin/DocumentLineageCard';
 import { billsService } from '../../../services/bills.service';
 import { formatMoney } from '../../../components/admin/LineItemsTable';
@@ -29,6 +29,9 @@ export const BillDetailPage: React.FC = () => {
 
   const [payDialogOpen, setPayDialogOpen] = useState(false);
   const [payAmount, setPayAmount] = useState('');
+  // Optional payment date — defaults to today, backdate it to when the
+  // payment actually arrived. Drives `paid_at` (cash-basis revenue windows).
+  const [payDate, setPayDate] = useState(new Date().toISOString().slice(0, 10));
   const [payMethod, setPayMethod] = useState('');
   const [payReference, setPayReference] = useState('');
   const [payNotes, setPayNotes] = useState('');
@@ -210,6 +213,7 @@ export const BillDetailPage: React.FC = () => {
     try {
       await billsService.markPaid(inv.id, {
         amountMinor: Math.round(Number(payAmount) * 100),
+        paidAt: payDate || undefined,
         paymentMethod: payMethod || undefined,
         reference: payReference || undefined,
         notes: payNotes || undefined,
@@ -218,6 +222,7 @@ export const BillDetailPage: React.FC = () => {
       setPayDialogOpen(false);
       setPayAmount(''); setPayMethod(''); setPayReference(''); setPayNotes('');
       setPayWithSkonto(false);
+      setPayDate(new Date().toISOString().slice(0, 10));
       qc.invalidateQueries({ queryKey: ['invoice', id] });
       toast.success(t('bills.paymentRecordedToast', 'Payment recorded.'));
     } catch (e: any) {
@@ -466,6 +471,14 @@ export const BillDetailPage: React.FC = () => {
             <div className="space-y-3">
               <Input type="number" step="0.01" label={t('bills.payment.amount', 'Amount') as string} value={payAmount}
                 onChange={(e) => setPayAmount(e.target.value)} placeholder={String(outstanding.toFixed(2))} />
+              {/* Optional payment date — drives `paid_at`, which the
+                  dashboard's cash-basis revenue windows key on. Defaults
+                  to today; backdate it to when the payment actually arrived. */}
+              <LocalizedDateInput
+                label={t('bills.payment.date', 'Payment date') as string}
+                value={payDate}
+                onChange={setPayDate}
+              />
               {/* Skonto checkbox (migration 126). Only surfaced when
                   the invoice's payment terms actually offer Skonto —
                   the backend resolves skontoPercent from the snapshot