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

hooks/use-keyboard-shortcuts.ts

@@ -19,6 +19,7 @@ export interface KeyboardShortcutHandlers {
   onDelete?: () => void;
   onMarkAsUnread?: () => void;
   onMarkAsRead?: () => void;
+  onToggleSpam?: () => void;
 
   // Global actions
   onCompose?: () => void;
@@ -180,6 +181,13 @@ export function useKeyboardShortcuts({
           }
           break;
 
+        case "!":
+          if (selectedEmailId) {
+            event.preventDefault();
+            h.onToggleSpam?.();
+          }
+          break;
+
         // Global actions
         case "c":
           event.preventDefault();
@@ -264,6 +272,7 @@ export const KEYBOARD_SHORTCUTS = {
     { key: "# / Del", description: "shortcuts.actions.delete" },
     { key: "u", description: "shortcuts.actions.mark_unread" },
     { key: "Shift + I", description: "shortcuts.actions.mark_read" },
+    { key: "!", description: "shortcuts.actions.toggle_spam" },
   ],
   global: [
     { key: "c", description: "shortcuts.global.compose" },