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/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 };
   },
 };