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/pages/budget.js

@@ -47,6 +47,7 @@ let state = {
   entries:     [],
   summary:     null,
   prevSummary: null, // Vormonat für Monatsvergleich
+  currency:    'EUR',
 };
 let _container = null;
 
@@ -55,7 +56,7 @@ let _container = null;
 // --------------------------------------------------------
 
 function formatAmount(n) {
-  return new Intl.NumberFormat('de-DE', { style: 'currency', currency: 'EUR' }).format(n);
+  return new Intl.NumberFormat(getLocale(), { style: 'currency', currency: state.currency }).format(n);
 }
 
 function formatMonthLabel(ym) {
@@ -104,6 +105,11 @@ export async function render(container, { user }) {
   const today = new Date();
   state.month = `${today.getFullYear()}-${String(today.getMonth() + 1).padStart(2, '0')}`;
 
+  try {
+    const prefsRes = await api.get('/preferences');
+    state.currency = prefsRes.data?.currency ?? 'EUR';
+  } catch (_) { /* Fallback auf EUR */ }
+
   container.innerHTML = `
     <div class="budget-page">
       <h1 class="sr-only">${t('budget.title')}</h1>

public/pages/settings.js

@@ -9,6 +9,21 @@ import { t, formatDate, formatTime } from '/i18n.js';
 import { esc } from '/utils/html.js';
 import '/components/oikos-locale-picker.js';
 
+const SUPPORTED_CURRENCIES = ['AUD', 'CAD', 'CHF', 'CZK', 'DKK', 'EUR', 'GBP', 'HUF', 'JPY', 'NOK', 'PLN', 'SEK', 'USD'];
+
+function buildCurrencyOptions(selected) {
+  const display = typeof Intl.DisplayNames !== 'undefined'
+    ? new Intl.DisplayNames([document.documentElement.lang || 'en'], { type: 'currency' })
+    : null;
+  return SUPPORTED_CURRENCIES
+    .map((code) => {
+      const label = display ? `${code} - ${display.of(code)}` : code;
+      const sel = code === selected ? ' selected' : '';
+      return `<option value="${code}"${sel}>${label}</option>`;
+    })
+    .join('');
+}
+
 /**
  * @param {HTMLElement} container
  * @param {{ user: object }} context
@@ -23,7 +38,7 @@ export async function render(container, { user }) {
   let users        = [];
   let googleStatus = { configured: false, connected: false, lastSync: null };
   let appleStatus  = { configured: false, lastSync: null };
-  let prefs        = { visible_meal_types: ['breakfast', 'lunch', 'dinner', 'snack'] };
+  let prefs        = { visible_meal_types: ['breakfast', 'lunch', 'dinner', 'snack'], currency: 'EUR' };
 
   try {
     const [usersRes, gStatus, aStatus, prefsRes] = await Promise.allSettled([
@@ -114,6 +129,18 @@ export async function render(container, { user }) {
         </div>
       </section>
 
+      <!-- Budget -->
+      <section class="settings-section">
+        <h2 class="settings-section__title">${t('settings.sectionBudget')}</h2>
+        <div class="settings-card">
+          <h3 class="settings-card__title">${t('settings.currencyLabel')}</h3>
+          <p class="form-hint" style="margin-bottom:var(--space-3)">${t('settings.currencyHint')}</p>
+          <select class="form-input" id="currency-select">
+            ${buildCurrencyOptions(prefs.currency)}
+          </select>
+        </div>
+      </section>
+
       <!-- Mein Konto -->
       <section class="settings-section">
         <h2 class="settings-section__title">${t('settings.sectionAccount')}</h2>
@@ -330,6 +357,19 @@ function bindEvents(container, user) {
     });
   }
 
+  // Währungs-Auswahl
+  const currencySelect = container.querySelector('#currency-select');
+  if (currencySelect) {
+    currencySelect.addEventListener('change', async () => {
+      try {
+        await api.put('/preferences', { currency: currencySelect.value });
+        window.oikos?.showToast(t('settings.currencySaved'), 'success');
+      } catch (err) {
+        window.oikos?.showToast(err.message ?? t('common.errorGeneric'), 'danger');
+      }
+    });
+  }
+
   // Passwort ändern
   const passwordForm = container.querySelector('#password-form');
   if (passwordForm) {