test: stabilize ThemeToggle localStorage unit tests

[stokelykent-stack]

Fix ThemeToggle unit tests that can fail in some JS DOM shims where window.localStorage is only partially implemented (missing clear() / removeItem()).

What changed

• Define a small Storage-like localStorage mock in tests/unit/themeToggle.test.ts via Object.defineProperty(window, "localStorage", …)
• Keeps the intent the same: persist theme on toggle + read stored theme on mount

Why

• npm test was failing with TypeError: window.localStorage.clear is not a function.