feat(accounting): scope the tax-report export to income-only or cost-only

Adds a Complete / Income only / Cost only selector to the readable PDF + CSV
export (the on-screen report stays complete). Income-only emits just the
outgoing rows + the income summary line (+ the per-rate breakdown in the PDF);
cost-only emits the incoming-invoice + expense rows + the cost line and drops
the income-by-rate breakdown. Useful in Liechtenstein where, under the income
threshold, a flat 20% Gewinnungskosten deduction is sometimes better than actual
costs — handing the Treuhänder just the income (or just the cost) basis is
cleaner.

Backend: renderTaxReportPdf/Csv take a `scope` param (all|income|cost) that
filters report.ledger by row.type + the summary lines; the /pdf + /csv routes
accept & validate `?scope=`; filenames get an income_/cost_ tag. Frontend:
scope <select> beside the export buttons, threaded through buildQueryString.
i18n en/de. The 20% calculation itself is intentionally NOT in-app (applied by
the Treuhänder) per the scoping decision.

frontend/src/i18n/locales/de.json

@@ -3953,6 +3953,10 @@
     "export": {
       "reportTitle": "Bericht",
       "reportHint": "Lesbare Liste — für Ihre Unterlagen.",
+      "scopeLabel": "Export-Umfang",
+      "scopeAll": "Vollständig",
+      "scopeIncome": "Nur Einnahmen",
+      "scopeCost": "Nur Kosten",
       "journalTitle": "Buchungsjournal"
     },
     "exportCsv": "CSV exportieren",

frontend/src/i18n/locales/en.json

@@ -3953,6 +3953,10 @@
     "export": {
       "reportTitle": "Report",
       "reportHint": "Readable list — for your own records.",
+      "scopeLabel": "Export scope",
+      "scopeAll": "Complete",
+      "scopeIncome": "Income only",
+      "scopeCost": "Cost only",
       "journalTitle": "Accounting journal"
     },
     "exportCsv": "Export CSV",

frontend/src/pages/admin/clients/TaxReportPage.tsx

@@ -102,6 +102,8 @@ export const TaxReportPage: React.FC = () => {
   const [to,   setTo]   = useState(initialPeriod.to);
   const [currency, setCurrency] = useState<string>('CHF');
   const [isExporting, setIsExporting] = useState<'pdf' | 'csv' | 'ledger' | null>(null);
+  // Export-only scope (income/cost split). The on-screen report stays complete.
+  const [exportScope, setExportScope] = useState<'all' | 'income' | 'cost'>('all');
   // Treuhänder (collective-journal) export — same period/currency as the
   // report; target tool picks the import format (generic / Banana / bexio).
   const [ledgerFormat, setLedgerFormat] = useState<ExportFormat>('generic');
@@ -132,9 +134,10 @@ export const TaxReportPage: React.FC = () => {
   const handleExport = async (format: 'pdf' | 'csv') => {
     setIsExporting(format);
     try {
+      const exportParams = { ...params, scope: exportScope };
       const { url, filename } = format === 'pdf'
-        ? await taxReportService.downloadPdfUrl(params)
-        : await taxReportService.downloadCsvUrl(params);
+        ? await taxReportService.downloadPdfUrl(exportParams)
+        : await taxReportService.downloadCsvUrl(exportParams);
       triggerBrowserDownload(url, filename);
     } catch (err) {
       toast.error(t('taxReport.exportFailed', 'Export failed. Please try again.'));
@@ -313,6 +316,17 @@ export const TaxReportPage: React.FC = () => {
                   </div>
                 </div>
                 <div className="flex flex-wrap items-center gap-2">
+                  <select
+                    value={exportScope}
+                    onChange={(e) => setExportScope(e.target.value as 'all' | 'income' | 'cost')}
+                    disabled={exportsDisabled}
+                    aria-label={t('taxReport.export.scopeLabel', 'Export scope')}
+                    className={`${selectClassName} min-w-[150px]`}
+                  >
+                    <option value="all">{t('taxReport.export.scopeAll', 'Complete')}</option>
+                    <option value="income">{t('taxReport.export.scopeIncome', 'Income only')}</option>
+                    <option value="cost">{t('taxReport.export.scopeCost', 'Cost only')}</option>
+                  </select>
                   <Button
                     className="min-w-[150px]"
                     variant="outline"

frontend/src/services/taxReport.service.ts

@@ -142,6 +142,9 @@ export interface TaxReportParams {
   to: string;
   currency: string;
   locale?: string;
+  /** Export-only scope: 'all' (default) | 'income' | 'cost'. Ignored by the
+   *  on-screen report; the PDF/CSV exports filter the ledger + summary. */
+  scope?: 'all' | 'income' | 'cost';
 }
 
 function buildQueryString(params: TaxReportParams): string {
@@ -151,6 +154,7 @@ function buildQueryString(params: TaxReportParams): string {
     currency: params.currency,
   });
   if (params.locale) usp.set('locale', params.locale);
+  if (params.scope && params.scope !== 'all') usp.set('scope', params.scope);
   return usp.toString();
 }
 
@@ -172,7 +176,8 @@ export const taxReportService = {
       responseType: 'blob',
     });
     const url = URL.createObjectURL(res.data);
-    const filename = `tax_report_${params.from}_to_${params.to}_${params.currency}.pdf`;
+    const scopeTag = params.scope && params.scope !== 'all' ? `${params.scope}_` : '';
+    const filename = `tax_report_${scopeTag}${params.from}_to_${params.to}_${params.currency}.pdf`;
     return { url, filename };
   },
 
@@ -181,7 +186,8 @@ export const taxReportService = {
       responseType: 'blob',
     });
     const url = URL.createObjectURL(res.data);
-    const filename = `tax_report_${params.from}_to_${params.to}_${params.currency}.csv`;
+    const scopeTag = params.scope && params.scope !== 'all' ? `${params.scope}_` : '';
+    const filename = `tax_report_${scopeTag}${params.from}_to_${params.to}_${params.currency}.csv`;
     return { url, filename };
   },
 };