[github-actions]

Closes #3

Ship summary: quick light/dark theme toggle

Round	Reviewer	QA
1	CHANGES (1: top bar overflows at 320px with "Sign in" pill)	PASS (9 new tests)
2	APPROVE	PASS (426 passed, 1 skipped)

Assumptions

• Icon shows the theme you will switch to (moon while light, sun while dark); aria-label "Switch to dark/light theme".
• Reuses icon-btn/account-btn classes for focus ring and 44px tap target.
• Toggling while on Settings remounts that page (key={theme}) so its select stays in sync; any sync message shown there is lost.
• 320px fit achieved by hiding the 日本語 label under 23rem. Estimated from CSS, NOT verified in a real browser (jsdom can't). Please check signed-out and signed-in at 320px.

Summary

Sun/moon button in the phone top bar (next to account) and in the wide-screen nav, hidden in full-screen lessons. Flips the displayed theme (System resolves via OS preference), saves to nihongo.theme like Settings. Committed, not pushed.

Run log: https://github.com/Prodyte/nihongo/actions/runs/38523062239