feat: add UI/UX polish with navigation rail, confirm dialogs, welcome banner, and form validation

- Add NavigationRail component (desktop vertical icon sidebar + mobile bottom tab bar)
- Add ConfirmDialog with promise-based useConfirmDialog hook for async confirmation flow
- Add WelcomeBanner onboarding component (one-time display, localStorage persistence)
- Polish login form UX (shake on error, TOTP slide animation, password visibility toggle, session expired banner)
- Add inline form validation with shake animation in email composer and contacts
- Add empty state patterns for contacts (no data vs no search results with contextual actions)
- Improve toast notification system with undo action support and typed durations
- Add WCAG AA prefers-reduced-motion media query, safe area insets, sr-only live regions
- Add template settings tab and keyboard shortcut integration
- Update all 8 locale translations

components/email/thread-conversation-view.tsx

@@ -297,6 +297,11 @@ function EmailCard({
             }
           }
 
+          if (node.tagName === 'A') {
+            node.setAttribute('target', '_blank');
+            node.setAttribute('rel', 'noopener noreferrer');
+          }
+
           if (resolvedTheme === 'dark') {
             if (htmlNode.style) {
               const originalStyles = htmlNode.style.cssText;