[WIP] Implement tasks from documentation TODO.md

[Copilot]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

I placed doc/SPEC.md for final goal.
To implement this app, see doc/TODO.md to recognize what needs to be done.
After reading these two files, please implement every items in todo list.

1. Write the specifications for the task you are working on in docs/specs/
2. Write the code for the task you are working on.
3. Write the test code for the task you are working on.
4. If all the tests pass (bun run test:all), mark item in doc/TODO.md as done.

Important Notes

• Please update @/docs/specs.md and docs/specs/ if you make changes to the project.
• Try to reuse codes if possible. Create @/docs/specs/utils.md for available reusable utility functions.

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