feat: expand internationalization and add identity management

This release significantly expands internationalization support and adds comprehensive identity management features.

Internationalization (i18n):
- Add 5 new languages: Spanish, Italian, German, Dutch, Portuguese
- Expand from 3 to 8 total supported languages
- Redesign language switcher for better scalability (dropdown UI)
- Complete translations for all features across all languages

Identity Management:
- Multiple sender identities with per-identity signatures
- Sub-addressing support (user+tag@domain.com)
- Context-aware tag suggestions for sub-addresses
- Identity badges in email viewer and list
- Full CRUD operations for managing identities

Newsletter Management:
- RFC 2369 List-Unsubscribe support (one-click unsubscribe)
- HTTP and mailto unsubscribe methods
- Security validation prevents XSS attacks
- Two-step confirmation with persistent dismissal

Security & Accessibility:
- Dark mode email readability (intelligent color transformation)
- WCAG 2.0 Level AA color contrast compliance
- Comprehensive XSS prevention with validation utilities
- Unit test coverage for security-critical code (57 validation tests)

Testing:
- Add unit tests for validation utilities
- Add unit tests for email sanitization
- Add unit tests for color transformation
- Full test coverage for XSS attack vectors

components/providers/intl-provider.tsx

@@ -5,11 +5,23 @@ import { NextIntlClientProvider } from 'next-intl';
 import { useLocaleStore } from '@/stores/locale-store';
 import enMessages from '@/locales/en/common.json';
 import frMessages from '@/locales/fr/common.json';
+import jaMessages from '@/locales/ja/common.json';
+import esMessages from '@/locales/es/common.json';
+import itMessages from '@/locales/it/common.json';
+import deMessages from '@/locales/de/common.json';
+import nlMessages from '@/locales/nl/common.json';
+import ptMessages from '@/locales/pt/common.json';
 
 // Pre-loaded translations (loaded at build time, not runtime)
 const ALL_MESSAGES = {
   en: enMessages,
   fr: frMessages,
+  ja: jaMessages,
+  es: esMessages,
+  it: itMessages,
+  de: deMessages,
+  nl: nlMessages,
+  pt: ptMessages,
 };
 
 interface IntlProviderProps {