[codex] Polish Kanji a Day product loop

[Alisa-Novik]

Summary

Turns the Kanji a Day preview into a more production-shaped consumer app experience while keeping the work scoped to the no-build PWA preview.

• Removes user-visible MVP/debug/implementation copy from normal mode.
• Keeps progress/debug controls behind ?debug=1.
• Tightens the watch-frame layout so header text and controls stay inside the rounded safe area at 360px, 390px, 430px, and desktop widths.
• Implements an explicit learning state flow: not started, practicing, stroke complete, meaning viewed, learned.
• Improves stroke playback with future/completed/current stroke layers, SVG draw-on animation, a subtle brush-tip marker, replay from stroke 1, completion pulse, and reduced-motion support.
• Expands local content to seven beginner kanji: 日, 月, 火, 水, 木, 金, 土.
• Adds product-mode home stats for Today, Streak, Learned, and Next.
• Adds history empty state, learned cards, and tappable history detail/meaning view.

Validation

• python3 HTML parse smoke check.
• git diff --check -- apps/watch/KanjiADay/Preview/index.html.
• Forbidden normal-mode copy scan: no MVP, PWA, AI Calls, Simulate, Reset, localhost, no backend, or no payments matches in the preview file.
• Playwright full flow: Today -> stroke playback -> Meaning -> Got it -> History -> History detail.
• Playwright persistence check after reload.
• Playwright debug gating check: debug panel hidden in normal mode and visible with ?debug=1.
• Playwright reduced-motion flow reaches Meaning without draw animation.
• Playwright button-bounds checks plus screenshots at 360px, 390px, 430px, and desktop.
• Browser console checked clean during the interactive flow.

Screenshots

Local validation screenshots were written under workspace/runs/kanji-a-day-prod-pass/:

• today-product-home-430.png
• stroke-complete-430.png
• meaning-430.png
• history-430.png
• history-detail-430.png
• desktop-home.png

Notes

The worktree contains unrelated pre-existing changes. This PR intentionally stages and commits only apps/watch/KanjiADay/Preview/index.html.