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/email/email-identity-badge.tsx

@@ -0,0 +1,142 @@
+'use client';
+
+import { useTranslations } from 'next-intl';
+import { Mail, Tag } from 'lucide-react';
+import { cn } from '@/lib/utils';
+import type { Email, Identity } from '@/lib/jmap/types';
+import { parseSubAddress } from '@/lib/sub-addressing';
+
+interface EmailIdentityBadgeProps {
+  email: Email;
+  identities: Identity[];
+  compact?: boolean;
+  className?: string;
+}
+
+export function EmailIdentityBadge({
+  email,
+  identities,
+  compact = false,
+  className,
+}: EmailIdentityBadgeProps) {
+  const t = useTranslations('identities.badge');
+
+  const fromAddress = email.from?.[0]?.email;
+  if (!fromAddress) return null;
+
+  // Parse the from address to check for sub-addressing
+  const parsedFrom = parseSubAddress(fromAddress);
+
+  // Find matching identity (email sent BY the user)
+  const matchingIdentity = identities.find(
+    (identity) => identity.email === fromAddress || identity.email === `${parsedFrom.baseUser}@${parsedFrom.domain}`
+  );
+
+  // Check if email was sent TO a sub-address (received email)
+  let receivedToTag: string | null = null;
+  if (!matchingIdentity) {
+    // Check all TO addresses for sub-address tags matching user's identities
+    for (const recipient of email.to || []) {
+      const parsedTo = parseSubAddress(recipient.email);
+      if (parsedTo.tag) {
+        // Check if this base email matches any of the user's identities
+        const matchingToIdentity = identities.find(
+          (identity) => identity.email === `${parsedTo.baseUser}@${parsedTo.domain}`
+        );
+        if (matchingToIdentity) {
+          receivedToTag = parsedTo.tag;
+          break;
+        }
+      }
+    }
+  }
+
+  // Determine which tag to display (sent or received)
+  const displayTag = matchingIdentity ? parsedFrom.tag : receivedToTag;
+
+  // Don't show badge if not from user's identity and not to user's sub-address
+  if (!matchingIdentity && !receivedToTag) return null;
+
+  if (compact) {
+    // Compact view for email list
+    if (displayTag) {
+      return (
+        <div
+          className={cn(
+            'inline-flex items-center gap-1 px-1.5 py-0.5 rounded text-xs',
+            'bg-primary/10 text-primary',
+            className
+          )}
+          title={t('sub_address_tag', { tag: displayTag })}
+        >
+          <Tag className="w-3 h-3" />
+          <span className="font-mono">+{displayTag}</span>
+        </div>
+      );
+    }
+
+    if (
+      matchingIdentity &&
+      matchingIdentity.name &&
+      matchingIdentity.name !== matchingIdentity.email &&
+      matchingIdentity.name !== fromAddress
+    ) {
+      return (
+        <div
+          className={cn(
+            'inline-flex items-center gap-1 px-1.5 py-0.5 rounded text-xs',
+            'bg-secondary text-muted-foreground',
+            className
+          )}
+          title={t('identity_name', { name: matchingIdentity.name })}
+        >
+          <Mail className="w-3 h-3" />
+          <span className="truncate max-w-[100px]">{matchingIdentity.name}</span>
+        </div>
+      );
+    }
+
+    return null;
+  }
+
+  // Full view for email viewer - now shows compact badges only
+  return (
+    <div className={cn('inline-flex items-center gap-2', className)}>
+      {/* Sub-address tag badge */}
+      {displayTag && (
+        <div
+          className={cn(
+            'inline-flex items-center gap-1 px-2 py-0.5 rounded-md',
+            'bg-primary/10 text-primary border border-primary/20',
+            'text-xs font-semibold'
+          )}
+          title={t('sub_address_tag', { tag: displayTag })}
+          aria-label={t('sub_address_tag', { tag: displayTag })}
+        >
+          <Tag className="w-3 h-3" />
+          <span className="font-mono">{t('subaddress_tag', { tag: displayTag })}</span>
+        </div>
+      )}
+
+      {/* Identity badge (only if identity has a name and no sub-address tag) */}
+      {!displayTag &&
+        matchingIdentity &&
+        matchingIdentity.name &&
+        matchingIdentity.name !== matchingIdentity.email &&
+        matchingIdentity.name !== fromAddress && (
+          <div
+            className={cn(
+              'inline-flex items-center gap-1 px-2 py-0.5 rounded-md',
+              'bg-secondary text-muted-foreground border border-border',
+              'text-xs font-medium'
+            )}
+            title={t('identity_name', { name: matchingIdentity.name })}
+            aria-label={t('identity_name', { name: matchingIdentity.name })}
+          >
+            <Mail className="w-3 h-3" />
+            <span>{t('identity_short', { name: matchingIdentity.name })}</span>
+          </div>
+        )}
+    </div>
+  );
+}