test: cover theme helpers

[gaoflow]

Summary

• add tests for readTheme, writeTheme, and effectiveTheme
• cover valid, missing, and invalid localStorage values
• cover explicit light/dark themes and system theme resolution through mocked matchMedia
• cover SSR-safe branches where window is unavailable

Closes #16

Tests

• npm test -- src/lib/__tests__/theme.test.ts --runInBand
• npm run lint
• npm test -- --runInBand
• npm test -- --coverage --runInBand
• npm run build

Coverage note: lib/theme.ts reports 100% statement, branch, function, and line coverage in the coverage run.

[mikewheeleer]

thanks for jumping in @gaoflow! 🙏 we assign issues before work starts so two people don't build the same thing, and #16 isn't assigned to you. please grab an open unassigned issue and PR against that. closing this one for now, hope to see you back 💪