feat: Add UML diagrams for login activity, flowchart, and sequence

docs/login/login-ac.puml

+@startuml
+title Login Activity
+
+start
+:User provides credentials;
+:System authenticates;
+if (Authentication successful?) then (yes)
+  :Grant access to system;
+  :Display main page;
+else (no)
+  :Display error message;
+endif
+:End;
+stop
+
+@enduml
+

docs/login/login-fc.puml

+@startuml
+title Login Flowchart
+
+start
+:User enters username and password;
+:System validates credentials;
+if (Credentials valid?) then (yes)
+  :Redirect to main page;
+  stop
+else (no)
+  :Display error message;
+  :Return to login page;
+  stop
+endif
+
+@enduml
+

docs/login/login-sq.puml

+@startuml
+title Login Sequence
+
+actor User
+participant "Login Screen" as Login
+participant "Authentication Service" as AuthService
+database "User Database" as DB
+
+User -> Login: Enters credentials
+Login -> AuthService: Sends credentials for verification
+AuthService -> DB: Queries user data
+DB --> AuthService: Returns user data
+
+alt Credentials match
+    AuthService -> Login: Authentication successful
+    Login -> User: Redirects to main page
+else Credentials do not match
+    AuthService -> Login: Authentication failed
+    Login -> User: Shows error message
+end
+
+@enduml
+