feat(budget): configurable currency in settings (#20)

Add household-wide currency preference for the budget section.
Users can select from 13 currencies (EUR, USD, GBP, SEK, NOK, DKK,
CHF, PLN, CZK, HUF, JPY, AUD, CAD) in Settings → Budget.

- preferences API (GET/PUT) now includes currency field
- budget page loads currency from preferences on render
- formatAmount() uses locale-aware Intl.NumberFormat with chosen currency
- settings page gains a Budget section with a currency select
- all three locales (de, en, it) updated with new i18n keys

public/locales/it.json

@@ -418,7 +418,7 @@
     "typeIncome": "Entrata",
     "titleLabel": "Titolo *",
     "titlePlaceholder": "es. Supermercato",
-    "amountLabel": "Importo (€) *",
+    "amountLabel": "Importo *",
     "amountPlaceholder": "0,00",
     "categoryLabel": "Categoria",
     "dateLabel": "Data *",
@@ -518,7 +518,11 @@
     "mealTypesLabel": "Pasti visibili",
     "mealTypesHint": "Solo i tipi di pasto selezionati vengono mostrati nel piano pasti.",
     "mealTypesSaved": "Impostazioni del piano pasti salvate.",
-    "mealTypesMinOne": "Almeno un tipo di pasto deve essere attivo."
+    "mealTypesMinOne": "Almeno un tipo di pasto deve essere attivo.",
+    "sectionBudget": "Bilancio",
+    "currencyLabel": "Valuta",
+    "currencyHint": "Imposta la valuta utilizzata in tutta la sezione budget.",
+    "currencySaved": "Valuta salvata."
   },
 
   "login": {