feat(accounting): move Treuhänder export onto the Tax page

The standalone 'Treuhänder export' tab duplicated the Tax page's period/
currency filters over the same data. Fold the collective-journal export into
the Tax page as a third export action (target-tool format picker: generic /
Banana / bexio), beside Export CSV/PDF, with a link to its Chart-of-accounts
config. Removes the Accounting sub-nav 'export' tab (old /export route now
redirects to the Tax page); keeps Chart of accounts as its own setup tab.
Deletes the now-orphaned LedgerExportPage.

Build + JSON parse green.

frontend/src/i18n/locales/de.json

@@ -3937,6 +3937,9 @@
       "currency": "Währung"
     },
     "exportPdf": "PDF exportieren",
+    "ledgerExport": "Treuhänder-Export",
+    "ledgerExportHint": "Doppelte Buchung für Ihren Treuhänder, abgebildet über Ihren Kontenplan.",
+    "ledgerExportConfigure": "Einrichten →",
     "exportCsv": "CSV exportieren",
     "exportFailed": "Export fehlgeschlagen. Bitte erneut versuchen.",
     "errorTitle": "Steuerliste konnte nicht geladen werden",

frontend/src/i18n/locales/en.json

@@ -3937,6 +3937,9 @@
       "currency": "Currency"
     },
     "exportPdf": "Export PDF",
+    "ledgerExport": "Treuhänder export",
+    "ledgerExportHint": "Double-entry journal for your accountant, mapped via your Chart of accounts.",
+    "ledgerExportConfigure": "Configure →",
     "exportCsv": "Export CSV",
     "exportFailed": "Export failed. Please try again.",
     "errorTitle": "Could not load tax report",