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').

backend/src/routes/adminDashboard.js

@@ -429,37 +429,25 @@ router.get('/crm-stats', adminAuth, async (req, res) => {
           if (r.status in invoiceCounts) invoiceCounts[r.status] = Number(r.count) || 0;
         }
 
-        // Revenue windows: sum of `paid_amount_minor` for invoices
-        // marked PAID inside the window. Using paid_amount (not total)
-        // so partial payments are tracked accurately. Stornos excluded
-        // — they're never status='paid' in normal flow but the guard
-        // is defensive.
+        // Revenue windows: sum of `paid_amount_minor` for invoices marked
+        // PAID whose payment date (`paid_at`) falls inside the window —
+        // cash-basis recognition (revenue counts when the money arrives).
+        // Using paid_amount (not total) so partial payments are tracked
+        // accurately. Stornos excluded — never status='paid' in normal flow,
+        // the guard is defensive.
         //
-        // Recognition date differs by origin:
-        //   • imported (historical) invoices → recognise on issue_date,
-        //     their true economic date. A year-old imported invoice must
-        //     never land in the rolling "last 30/90 days" window. This is
-        //     robust even for rows imported BEFORE the import route began
-        //     anchoring paid_at to issue_date (commit c6b8cc9) — those
-        //     legacy rows still carry an import-time paid_at, so keying on
-        //     issue_date is what actually fixes them.
-        //   • native invoices → recognise on paid_at (cash-basis).
+        // `paid_at` is admin-controllable, so an old/imported invoice lands
+        // in the right window without any special-casing here: the mark-paid
+        // dialog takes an optional payment date (backdate to when the money
+        // actually arrived) and the historical-import route anchors paid_at
+        // to the invoice's issue_date.
         const winSum = async (cutoff) => {
-          const cutoffDateStr = cutoff.toISOString().slice(0, 10);
           const row = await db('invoices')
             .where('status', 'paid')
+            .where('paid_at', '>=', cutoff)
             .andWhere(function() {
               this.whereNot('kind', 'storno').orWhereNull('kind');
             })
-            .andWhere(function() {
-              this.where(function() {
-                this.whereNotNull('imported_pdf_path')
-                  .andWhere('issue_date', '>=', cutoffDateStr);
-              }).orWhere(function() {
-                this.whereNull('imported_pdf_path')
-                  .andWhere('paid_at', '>=', cutoff);
-              });
-            })
             .sum('paid_amount_minor as total')
             .first();
           return Number(row?.total || 0);