feat: username/password authentication for admin and merchant panels

- Add PasswordHasher utility (PBKDF2/SHA-256, 100k iterations)
- Add Username + PasswordHash fields to Employee and SystemAdmin entities
- EF migration: AddPasswordLogin (nullable columns on both tables)
- Meezi.API: POST /api/auth/login (employee password login, CHOOSE_CAFE support)
- Meezi.API: PUT/DELETE /api/cafes/{id}/employees/{id}/credentials (Owner/Manager only)
- Meezi.Admin.API: POST /api/admin/auth/login + PUT /api/admin/auth/password
- Dashboard login page: OTP / Password tabs
- Admin login page: OTP / Password tabs
- HR screen: new Credentials tab for setting employee username/password
- PlatformDataSeeder: ensure system admin + integration settings in production
- Trial countdown banner: updated deadline to 1 Tir 1405 (Jun 22)
- i18n: fa/en/ar updated for all new UI strings

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/Meezi.Core/Entities/Employee.cs

@@ -12,6 +12,12 @@ public class Employee : TenantEntity
     public decimal BaseSalary { get; set; }
     public string? PinCode { get; set; }
 
+    /// <summary>Optional username for password-based dashboard/POS login (set by cafe admin).</summary>
+    public string? Username { get; set; }
+
+    /// <summary>PBKDF2/SHA-256 hash. Null means password login is not enabled for this employee.</summary>
+    public string? PasswordHash { get; set; }
+
     public Cafe Cafe { get; set; } = null!;
     public Branch? Branch { get; set; }
     public ICollection<Order> Orders { get; set; } = [];