feat: Transfer-Schlagwort mit festem Zielkonto (z. B. REVOLUT -> Konto Revolut)

- Transfer-Regel kann direkt ein Zielkonto tragen (target_account_id); ohne ->
  weiterhin Auto-Erkennung aus dem Buchungstext (Konto-Texte)
- Migration 0010; Endpunkt aktualisiert bestehendes Wort statt Konflikt
- Import bevorzugt das feste Zielkonto, sonst _guess_target_account
- Konten-Seite: Transfer-Erkennung mit Zielkonto-Auswahl + Anzeige am Chip
- Test: festes Zielkonto greift ohne Konto-Texte (75 grün)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

web/src/locales/en.json

@@ -173,8 +173,9 @@
   },
   "transferRules": {
     "title": "Transfer detection",
-    "hint": "Transactions whose text contains one of these words are recognized as a transfer. The target account is guessed from the booking text via each account's detection texts (edit account, kind Text).",
+    "hint": "Transactions whose text contains one of these words (e.g. “Revolut”) are set as a transfer. Pick a fixed target account – or let it be guessed automatically from the booking text (via each account's detection texts: edit account, kind “Text”).",
     "wordPlaceholder": "Word (e.g. Transfer)",
+    "autoTarget": "automatic",
     "empty": "No transfer keywords yet."
   },
   "categories": {