Add theme helper and ThemeToggle tests

[Errordog2]

Closes #16.

Summary

• Add focused Jest coverage for src/lib/theme.ts read/write/effective-theme helpers.
• Add Testing Library coverage for ThemeToggle rendering, aria-pressed state, persisted selections, and documentElement.dark class changes.
• Mock window.matchMedia in the tests so jsdom can exercise system theme behavior.

Validation

• Static inspection against the existing Jest/Testing Library setup.
• Tests are scoped to the requested files and do not add runtime dependencies.

No payment details are posted publicly.

[mikewheeleer]

clean little suite for the theme helper and ThemeToggle, thanks for this. lgtm 👍

[Errordog2]

Thanks for the quick review and merge. Since issue #16 is part of the GrantFox OSS / maybe rewarded campaign, is there any reward-tracking, rating, or confirmation step I should complete after merge?