feat: multiplayer todos

[adnjoo]

#104

Summary of Pull Request

Changed Files:

1. app/models/note.rb
   • Added methods for managing collaborators (add/remove).
   • Modified the association to belongs_to :owner, class_name: "User", foreign_key: :user_id.
   • Added has_many :notes_users and has_many :users, through: :notes_users.
2. app/models/notes_user.rb
   • New model created with associations to Note and User.
3. app/models/user.rb
   • Updated associations to include has_many :notes_users and has_many :notes, through: :notes_users.
   • Added has_many :owned_notes, class_name: "Note", foreign_key: :user_id.