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

lib/email-headers.ts

@@ -1,4 +1,5 @@
 import { AuthenticationResults } from './jmap/types';
+import { parseUnsubscribeUrls } from './validation';
 
 /**
  * Parse Authentication-Results header to extract SPF, DKIM, DMARC results
@@ -191,7 +192,11 @@ export function parseSpamLLM(header: string): { verdict: string; explanation: st
  */
 interface ListHeaders {
   listId?: string;
-  listUnsubscribe?: string;
+  listUnsubscribe?: {
+    http?: string;
+    mailto?: string;
+    preferred?: 'http' | 'mailto';
+  };
   listHelp?: string;
   listPost?: string;
 }
@@ -209,9 +214,11 @@ export function extractListHeaders(headers: Record<string, string | string[]>):
     const unsub = Array.isArray(headers['List-Unsubscribe'])
       ? headers['List-Unsubscribe'][0]
       : headers['List-Unsubscribe'];
-    // Extract URL from <url> format
-    const match = unsub.match(/<([^>]+)>/);
-    result.listUnsubscribe = match ? match[1] : unsub;
+
+    const parsed = parseUnsubscribeUrls(unsub);
+    if (parsed.preferred) {
+      result.listUnsubscribe = parsed;
+    }
   }
 
   if (headers['List-Help']) {