feat: add Japanese, Arabic, Hindi, Portuguese locales + new currencies

- 4 new locale files (ja, ar, hi, pt) with 567 keys each - full coverage
- Japanese (日本語): Hiragana/Kanji script
- Arabic (العربية): RTL-ready text
- Hindi (हिन्दी): Devanagari script
- Portuguese (Português): Brazilian Portuguese
- SUPPORTED_LOCALES updated in i18n.js (10 → 14 locales)
- LOCALE_LABELS updated in oikos-locale-picker.js
- New currencies: AED, BRL, INR, SAR added to budget settings
- Service Worker v31: new locale files pre-cached in APP_SHELL
- Docs: README, SPEC.md, BACKLOG.md, CHANGELOG.md updated

public/sw.js

@@ -12,7 +12,7 @@
  *   API: Immer Netzwerk (kein Caching von Nutzerdaten)
  */
 
-const SHELL_CACHE   = 'oikos-shell-v30';
+const SHELL_CACHE   = 'oikos-shell-v31';
 const PAGES_CACHE   = 'oikos-pages-v28';
 const ASSETS_CACHE  = 'oikos-assets-v27';
 const ALL_CACHES    = [SHELL_CACHE, PAGES_CACHE, ASSETS_CACHE];
@@ -27,6 +27,10 @@ const APP_SHELL = [
   '/rrule-ui.js',
   '/locales/de.json',
   '/locales/en.json',
+  '/locales/ja.json',
+  '/locales/ar.json',
+  '/locales/hi.json',
+  '/locales/pt.json',
   '/sw-register.js',
   '/lucide.min.js',
   '/styles/tokens.css',