feat(accounting): data-driven revenue-rate VAT map (multi-country)

The "VAT code by revenue rate" rows were hardcoded to the Swiss/LI rates
(8.1/2.6/3.8/0), so a code at any other rate (e.g. DE 19%/7%) had no row
to map. Derive the rows from the distinct rates of the OUTPUT VAT codes
instead — retype a code to a local rate and its row appears automatically;
remove the last code at a rate and the row drops. The CH/LI seeds are
unchanged and still produce the same four rows.

Frontend rateKey() mirrors backend ledgerService.rateKey so the saved map
keys keep matching the export-time lookup. Each rate's dropdown is scoped
to output codes at that rate. Empty state when no output codes exist.

frontend/src/i18n/locales/de.json

@@ -3903,7 +3903,7 @@
       "ledger_account_rebilled_revenue": "Weiterverrechnete Spesen (Ertrag)"
     },
     "vatMap": { "title": "MWST-Code nach steuerlicher Behandlung (Kosten)" },
-    "outputVatMap": { "title": "MWST-Code nach Umsatzsatz" },
+    "outputVatMap": { "title": "MWST-Code nach Umsatzsatz", "empty": "Fügen Sie oben Ausgangs-MWST-Codes hinzu, um pro Umsatzsatz einen Code festzulegen." },
     "categoryMap": { "title": "Aufwandkategorie → Konto" },
     "accounts": { "title": "Kontenplan" },
     "vatCodes": { "title": "MWST-Codes" },

frontend/src/i18n/locales/en.json

@@ -3903,7 +3903,7 @@
       "ledger_account_rebilled_revenue": "Re-billed expenses (revenue)"
     },
     "vatMap": { "title": "VAT code by tax treatment (costs)" },
-    "outputVatMap": { "title": "VAT code by revenue rate" },
+    "outputVatMap": { "title": "VAT code by revenue rate", "empty": "Add output VAT codes above to configure a code per revenue rate." },
     "categoryMap": { "title": "Expense category → account" },
     "accounts": { "title": "Chart of accounts" },
     "vatCodes": { "title": "VAT codes" },