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

stores/auth-store.ts

@@ -2,6 +2,7 @@ import { create } from 'zustand';
 import { persist } from 'zustand/middleware';
 import { JMAPClient } from '@/lib/jmap/client';
 import { useEmailStore } from './email-store';
+import { useIdentityStore } from './identity-store';
 import type { Identity } from '@/lib/jmap/types';
 
 interface AuthState {
@@ -46,6 +47,9 @@ export const useAuthStore = create<AuthState>()(
           const identities = await client.getIdentities();
           const primaryIdentity = identities.length > 0 ? identities[0] : null;
 
+          // Sync identities to identity store
+          useIdentityStore.getState().setIdentities(identities);
+
           // Success - save state (but NOT the password)
           set({
             isAuthenticated: true,
@@ -117,6 +121,9 @@ export const useAuthStore = create<AuthState>()(
           searchQuery: "",
           quota: null,
         });
+
+        // Clear identity store state
+        useIdentityStore.getState().clearIdentities();
       },
 
       checkAuth: async () => {