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/email-list-item.tsx

@@ -100,7 +100,7 @@ export function EmailListItem({ email, selected, onClick, onContextMenu }: Email
         <button
           onClick={handleCheckboxClick}
           className={cn(
-            "p-1 rounded mt-2 flex-shrink-0 transition-all duration-200",
+            "p-3 lg:p-1 rounded mt-2 flex-shrink-0 transition-all duration-200",
             "hover:bg-muted/50 hover:scale-110",
             "active:scale-95",
             isChecked && "text-primary"