feat: user profile self-service + admin password reset

- Add PUT /api/auth/profile for users to change their own email/password
- Add POST /api/admin/users/:id/reset-password for admin password resets
- Add updateProfileSchema and resetPasswordSchema validation (Zod)
- Create Profile.jsx page with email change and password change forms
- Add Reset Password button with inline form to ManageUsers in Admin panel
- Add /profile route and Account nav link in Layout

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/Layout.jsx

@@ -1,7 +1,7 @@
 import { Outlet, NavLink, useLocation } from 'react-router-dom';
 import { useAuth } from '../contexts/AuthContext';
 import ThemeToggle from './ThemeToggle';
-import { Calendar, Clock, Shield, LogOut, Menu, X } from 'lucide-react';
+import { Calendar, Clock, Shield, LogOut, Menu, X, User } from 'lucide-react';
 import { useState } from 'react';
 
 export default function Layout() {
@@ -12,6 +12,7 @@ export default function Layout() {
   const navItems = [
     { to: '/', icon: Clock, label: 'Timesheet' },
     { to: '/history', icon: Calendar, label: 'History' },
+    { to: '/profile', icon: User, label: 'Account' },
     ...(isAdmin ? [{ to: '/admin', icon: Shield, label: 'Admin' }] : []),
   ];