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

package.json

@@ -1,6 +1,6 @@
 {
   "name": "oikos",
-  "version": "0.11.1",
+  "version": "0.11.2",
   "description": "Self-hosted family planner - calendar, tasks, shopping, meal planning, budget and more. Private, open-source, no subscription.",
   "main": "server/index.js",
   "type": "module",