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/components/oikos-locale-picker.js

@@ -18,6 +18,10 @@ const LOCALE_LABELS = {
   ru: 'Русский',
   tr: 'Türkçe',
   zh: '中文',
+  ja: '日本語',
+  ar: 'العربية',
+  hi: 'हिन्दी',
+  pt: 'Português',
 };
 
 class OikosLocalePicker extends HTMLElement {