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/settings/settings-section.tsx

@@ -111,7 +111,8 @@ export function Select({ value, onChange, options }: SelectProps) {
     <select
       value={value}
       onChange={(e) => onChange(e.target.value)}
-      className="px-3 py-1.5 text-sm rounded bg-muted border border-border text-foreground focus:outline-none focus:ring-2 focus:ring-primary"
+      dir="auto"
+      className="px-3 py-1.5 text-sm rounded bg-muted border border-border text-foreground focus:outline-none focus:ring-2 focus:ring-primary cursor-pointer"
     >
       {options.map((option) => (
         <option key={option.value} value={option.value}>