Code Quality Check

# Required changes for better code quality
- It will be better to declare all variables at the top to avoid unwanted re-declarations. ([index.js#L72-73](https://github.com/IndieCoderMM/todo-app/blob/master/src/index.js#L72-L73))
- All modules should be in the `modules/` folder to provide a well-organized project.
- Functions should do only one thing. Therefore, these conditions should be checked before calling the functions. ([task-manager.js#L10](https://github.com/IndieCoderMM/todo-app/blob/master/src/task-manager.js#L10), [task-manager.js#L18](https://github.com/IndieCoderMM/todo-app/blob/master/src/task-manager.js#L18), [update-status.js#L2](https://github.com/IndieCoderMM/todo-app/blob/master/src/update-status.js#L2))
- Code will be shorter and cleaner if `forEach` loops are used instead of regular `for` loops. ([task-manager.js#L11](https://github.com/IndieCoderMM/todo-app/blob/master/src/task-manager.js#L11))

Review src/index.js, src/task-manager.js, and src/update-status.js, beginning with the linked lines and the current module layout. Apply the requested variable, module-organization, function-responsibility, and loop-style changes across the named files. Done means the listed code-quality concerns are addressed without changing the task app's behavior.

Written by the indexing model from the issue text.