Add unit tests system

[HanXHX]

• Add helpers in Makefile
• Setup CI for unit tests
• Global add tests

[Copilot]

Pull request overview

This PR adds a unit testing setup across core packages of the CLI, plus local/CI coverage reporting, to improve confidence in API/auth/crypto/config behavior.

Changes:

• Added unit tests for UI formatting, crypto (AGE), config/token persistence, OIDC auth flows, and API client endpoints.
• Added a make test-coverage helper and ignored generated coverage artifacts.
• Updated CI to print a coverage summary from coverage.out.

Reviewed changes

Copilot reviewed 12 out of 13 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
internal/ui/format_test.go	Adds coverage for FormatSize human-readable byte formatting.
internal/crypto/age_test.go	Adds tests for hybrid AGE key parsing + encrypt/decrypt helpers and edge cases.
internal/config/config_test.go	Adds tests for viper defaults and token save/load/delete behavior and permissions.
internal/auth/oidc_test.go	Adds tests for OIDC device-flow helpers, refresh/revoke, and token source behavior.
internal/api/client_test.go	Adds tests for core HTTP client verbs, error handling, multipart upload, and UA transport.
internal/api/login_test.go	Adds tests for fetching OIDC config + discovery error cases.
internal/api/transfer_test.go	Adds tests for transfer listing/details/files and create/complete/enable/disable/chunk ops.
internal/api/user_test.go	Adds tests for user profile/quota and active key (including null) behavior.
cmd/transfer.go	Minor refactor in generateTransferPassphrase variable naming and whitespace cleanup.
cmd/helpers_test.go	Adds tests for CLI helpers (expiry formatting, passphrase generator, JWT offline detection).
Makefile	Adds test-coverage target (coverage.out/html generation + summary).
.gitignore	Ignores coverage.out and coverage.html.
.github/workflows/_ci.yml	Prints coverage summary after tests in CI.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

You can also share your feedback on Copilot code review. Take the survey.