Add locale-specific internationalization features

[newmattock]

Closes #445.

Summary

• add locale profiles for the supported languages with region, currency, time zone, direction, cultural color/symbol metadata, and regional content
• expose locale-aware date, number, and currency formatting through the i18n provider and translation hook
• persist full locale selection, apply html lang/dir/data attributes, and expand the preferences language selector with live locale previews
• add locale validation and RTL/formatting test coverage

Verification

• git diff --check
• npx --yes -p vitest@3.2.4 vitest run src/lib/__tests__/localeFeatures.test.ts --config /tmp/vitest.locale.config.mjs
• npx --yes -p typescript@6.0.3 tsc --ignoreConfig --noEmit --skipLibCheck --target ES2020 --lib ES2020,DOM --module ESNext --moduleResolution bundler src/lib/localeFeatures.ts
• npx --yes -p esbuild@0.24.2 esbuild src/components/I18nProvider.jsx --bundle --platform=browser --format=esm --loader:.json=json --external:react --external:react/jsx-runtime --external:react-i18next --external:i18next --external:i18next-browser-languagedetector --outfile=/tmp/i18nProvider.bundle.js
• npx --yes -p esbuild@0.24.2 esbuild src/components/preferences/UserPreferences.jsx --bundle --platform=browser --format=esm --external:react --external:react/jsx-runtime --external:lucide-react --external:../I18nProvider.jsx --external:../../hooks/usePreferences --external:./AddressBook --external:./ThemeSettings --external:./AccessibilitySettings --external:../notifications/NotificationPreferences --external:../../utils/offline --outfile=/tmp/userPreferences.bundle.js

Note: full npm ci/repo test execution remains blocked by existing package metadata: TypeScript 6 conflicts with react-i18next optional peer requirements, and @tensorflow/tfjs-node@^5.0.0 has no matching published version. The provider bundle also reports pre-existing duplicate compare keys in several translation JSON files.

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