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/en.json

@@ -418,7 +418,7 @@
     "typeIncome": "Income",
     "titleLabel": "Title *",
     "titlePlaceholder": "e.g. Supermarket",
-    "amountLabel": "Amount (€) *",
+    "amountLabel": "Amount *",
     "amountPlaceholder": "0.00",
     "categoryLabel": "Category",
     "dateLabel": "Date *",
@@ -518,7 +518,11 @@
     "mealTypesLabel": "Visible meals",
     "mealTypesHint": "Only selected meal types are shown in the meal planner.",
     "mealTypesSaved": "Meal plan settings saved.",
-    "mealTypesMinOne": "At least one meal type must be active."
+    "mealTypesMinOne": "At least one meal type must be active.",
+    "sectionBudget": "Budget",
+    "currencyLabel": "Currency",
+    "currencyHint": "Sets the currency used throughout the budget section.",
+    "currencySaved": "Currency saved."
   },
 
   "login": {