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/thread-list-item.tsx

@@ -9,6 +9,7 @@ import { useSettingsStore } from "@/stores/settings-store";
 import { useUIStore } from "@/stores/ui-store";
 import { getThreadColorTag } from "@/lib/thread-utils";
 import { ThreadEmailItem } from "./thread-email-item";
+import { useTranslations } from "next-intl";
 
 interface ThreadListItemProps {
   thread: ThreadGroup;
@@ -44,6 +45,7 @@ export function ThreadListItem({
   onContextMenu,
   onOpenConversation,
 }: ThreadListItemProps) {
+  const t = useTranslations('threads');
   const showPreview = useSettingsStore((state) => state.showPreview);
   const isMobile = useUIStore((state) => state.isMobile);
   const { latestEmail, participantNames, hasUnread, hasStarred, hasAttachment, emailCount } = thread;
@@ -235,7 +237,7 @@ export function ThreadListItem({
           {isLoading ? (
             <div className="py-4 flex items-center justify-center text-sm text-muted-foreground">
               <Loader2 className="w-4 h-4 animate-spin mr-2" />
-              Loading conversation...
+              {t('loading')}
             </div>
           ) : (
             emailsToShow.map((email, index) => (