Refactor add command: Separate concerns into modular architecture

[MrKeiKun]

Summary

This PR refactors the add.go command to address increasing complexity by separating concerns into a well-organized modular architecture.

• core.go - Core file processing logic
• space.go - Space calculations and reporting
• files.go - File operations and validation
• config.go - Configuration management
• ui.go - UI and progress handling

and of course, simplified cmd/add.go as the one handling calls to different modules. I didn't also forget about test file.

• add/files_test.go - Cleaned-up its test since there way duplicated ones.
• cmd/add_test.go - just the one calling the tests.

AI Assistance

• I used AI assistance (generate code, write tests, refactor, or docs)
  • If yes, briefly describe where and how:

Scope & Risk

• Adds tests or updates existing tests
• Security/privacy reviewed where applicable
• Performance/complexity considered

Hacktoberfest Notes

• Link the issue this PR resolves (required for consideration): Refactoring of add.go due to increasing complexity #98
• Explain the problem and your approach in your own words (1-3 sentences): I just went with the conventional way of splitting them.

Checklist

• PR title is descriptive
• Commits are meaningful (no bulk AI dumps)
• No generated files or lockfile-only changes unless justified

[codecov]

Codecov Report

❌ Patch coverage is 6.33245% with 355 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
internal/add/ui.go	0.00%	107 Missing ⚠️
internal/add/core.go	0.00%	75 Missing ⚠️
internal/add/config.go	0.00%	52 Missing ⚠️
internal/add/files.go	32.43%	50 Missing ⚠️
internal/add/space.go	0.00%	36 Missing ⚠️
cmd/add.go	0.00%	35 Missing ⚠️

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