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

lib/email-sanitization.ts

@@ -8,7 +8,7 @@ import DOMPurify from 'dompurify';
  */
 export const EMAIL_SANITIZE_CONFIG = {
   ADD_TAGS: [],
-  ADD_ATTR: ['target', 'style', 'class', 'width', 'height', 'align', 'valign', 'bgcolor', 'color'],
+  ADD_ATTR: ['target', 'rel', 'style', 'class', 'width', 'height', 'align', 'valign', 'bgcolor', 'color'],
   ALLOW_DATA_ATTR: false,
   FORCE_BODY: true,
   FORBID_TAGS: [