Add project files.

CalendarReminder.Domain/Entities/Reminder.cs

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+namespace CalendarReminder.Domain.Entities;
+
+public class Reminder
+{
+    public int Id { get; set; }
+    public string Title { get; set; } = "";
+    public string? Description { get; set; }
+    public DateTime ReminderDate { get; set; }
+    public bool IsCompleted { get; set; }
+    public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+    public bool NotificationSent { get; set; }
+
+    // FK
+    public int UserId { get; set; }
+    public User? User { get; set; }
+    public ICollection<ReminderShare> Shares { get; set; } = [];
+}

CalendarReminder.Domain/Entities/ReminderShare.cs

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+namespace CalendarReminder.Domain.Entities;
+
+public class ReminderShare
+{
+    public int Id { get; set; }
+    public int ReminderId { get; set; }
+    public Reminder? Reminder { get; set; }
+    public int SharedWithUserId { get; set; }
+    public User? SharedWithUser { get; set; }
+}

CalendarReminder.Domain/Entities/User.cs

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+namespace CalendarReminder.Domain.Entities;
+
+public class User
+{
+    public int Id { get; set; }
+    public string Username { get; set; } = "";
+    public string Email { get; set; } = "";
+    public string PasswordHash { get; set; } = "";
+    public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+
+    public ICollection<Reminder> Reminders { get; set; } = [];
+}