feat(accounting): tax window shows all costs (incoming invoices + expenses) alongside revenue

Einnahmen-Ausgaben view for the Milchbüchlein/simple-accounting case:
- taxReportService.getTaxReport now returns a cost side (loadCosts:
  incoming invoices + internal expenses, company- or event-booked,
  schema-guarded) plus a summary (income / costs / result, VAT payable)
- declined/duplicate costs excluded; re-billed costs kept (matching
  re-bill revenue is counted, so the net is correct)
- CSV + PDF exports gain a Costs section and an income/costs/result
  summary; pdf-i18n keys added for all 6 locales (fr/nl/pt/ru machine —
  flag for native review)
- frontend tax page renders the summary card, a costs table (company
  vs event), and a 'verify with Treuhänder' disclaimer
- tax-report tests cover the cost aggregation + zeroed summary when the
  accounting tables are absent; adminCrmAuth test enables the accounting
  master flag the route now requires

fr/nl/pt/ru strings are machine-generated and need native review.

frontend/src/i18n/locales/de.json

@@ -3769,6 +3769,25 @@
     "grandTotalVat": "Gesamt MwSt.",
     "grandTotalGross": "Gesamt Brutto",
     "cancelledFootnote": "{{count}} stornierte Rechnung(en) — Beträge nicht in den Summen enthalten (für lückenlose Nummernfolge dargestellt).",
+    "skontoTooltip": "Mit Skonto bezahlt",
+    "costsTitle": "Kosten (Eingangsrechnungen + Spesen)",
+    "costsDisclaimer": "Diese Einnahmen-Ausgaben-Übersicht ist eine Orientierungshilfe für Ihre Aufzeichnungen (Milchbüchleinrechnung). Vorsteuerabzug und Ergebnis hängen von der steuerlichen Behandlung jeder Kostenposition ab — vor der Einreichung mit Ihrem Treuhänder / der Steuerverwaltung prüfen.",
+    "summary": {
+      "title": "Einnahmen / Ausgaben",
+      "income": "Einnahmen",
+      "costs": "Ausgaben",
+      "result": "Ergebnis",
+      "vatPayable": "MWST-Zahllast (Umsatz- − Vorsteuer)"
+    },
+    "cost": {
+      "source": "Art",
+      "sourceIncoming": "Eingang",
+      "sourceExpense": "Spese",
+      "supplier": "Lieferant / Beschreibung",
+      "taxTreatment": "Steuerliche Behandlung",
+      "company": "Unternehmen",
+      "total": "Summe Kosten"
+    },
     "col": {
       "date": "Datum",
       "invoice": "Rechnung",
@@ -3777,7 +3796,8 @@
       "vatRate": "MwSt %",
       "net": "Netto",
       "vat": "MwSt.",
-      "total": "Brutto"
+      "total": "Brutto",
+      "skonto": "Skonto"
     }
   },
   "quotes": {

frontend/src/i18n/locales/en.json

@@ -3769,6 +3769,25 @@
     "grandTotalVat": "Total VAT",
     "grandTotalGross": "Total gross",
     "cancelledFootnote": "{{count}} cancelled invoice(s) — amounts excluded from totals (shown for audit-trail continuity).",
+    "skontoTooltip": "Paid with Skonto",
+    "costsTitle": "Costs (incoming invoices + expenses)",
+    "costsDisclaimer": "This income/expense overview is a guideline for your records (Einnahmen-Ausgaben-Rechnung). VAT reclaimability and the result figure depend on each cost’s tax treatment — verify with your Treuhänder / tax authority before filing.",
+    "summary": {
+      "title": "Income / costs",
+      "income": "Income",
+      "costs": "Costs",
+      "result": "Result",
+      "vatPayable": "VAT payable (output − input)"
+    },
+    "cost": {
+      "source": "Type",
+      "sourceIncoming": "Incoming",
+      "sourceExpense": "Expense",
+      "supplier": "Supplier / description",
+      "taxTreatment": "Tax treatment",
+      "company": "Company",
+      "total": "Total costs"
+    },
     "col": {
       "date": "Date",
       "invoice": "Invoice",
@@ -3777,7 +3796,8 @@
       "vatRate": "VAT %",
       "net": "Net",
       "vat": "VAT",
-      "total": "Gross"
+      "total": "Gross",
+      "skonto": "Skonto"
     }
   },
   "quotes": {