feat(accounting): clearer tax-export window + gate journal export on accounting flag

- Restructure the export area into two labelled groups: 'Report' (PDF/CSV,
  for you) and 'Accounting journal' (for your accountant), each with a
  one-line caption — instead of two unlabelled button rows.
- i18n: the English label was the German 'Treuhänder export' → now 'Accountant
  export' (de stays 'Treuhänder-Export'); hint reworded.
- Feature flags: the journal export is an accounting-layer feature (needs the
  Chart-of-accounts mapping), so gate it on the 'accounting' master — the
  group only renders when accounting is on, and the backend /export route no
  longer requires the 'taxReport' sub-flag (the router already requires accounting).

Build + node --check + JSON parse green.

frontend/src/i18n/locales/de.json

@@ -3938,8 +3938,13 @@
     },
     "exportPdf": "PDF exportieren",
     "ledgerExport": "Treuhänder-Export",
-    "ledgerExportHint": "Doppelte Buchung für Ihren Treuhänder, abgebildet über Ihren Kontenplan.",
+    "ledgerExportHint": "Doppelte Buchungssätze für Ihren Treuhänder, abgebildet über Ihren Kontenplan.",
     "ledgerExportConfigure": "Einrichten →",
+    "export": {
+      "reportTitle": "Bericht",
+      "reportHint": "Lesbare Liste — für Ihre Unterlagen.",
+      "journalTitle": "Buchungsjournal"
+    },
     "exportCsv": "CSV exportieren",
     "exportFailed": "Export fehlgeschlagen. Bitte erneut versuchen.",
     "errorTitle": "Steuerliste konnte nicht geladen werden",

frontend/src/i18n/locales/en.json

@@ -3937,9 +3937,14 @@
       "currency": "Currency"
     },
     "exportPdf": "Export PDF",
-    "ledgerExport": "Treuhänder export",
-    "ledgerExportHint": "Double-entry journal for your accountant, mapped via your Chart of accounts.",
+    "ledgerExport": "Accountant export",
+    "ledgerExportHint": "Double-entry postings for your accountant, mapped via your Chart of accounts.",
     "ledgerExportConfigure": "Configure →",
+    "export": {
+      "reportTitle": "Report",
+      "reportHint": "Readable list — for your own records.",
+      "journalTitle": "Accounting journal"
+    },
     "exportCsv": "Export CSV",
     "exportFailed": "Export failed. Please try again.",
     "errorTitle": "Could not load tax report",