feat: add Vercel Web Analytics and update README

[Jiseoup]

What & Why

Adds Vercel Web Analytics to measure traffic on the live demo.

Since this is a privacy-focused, self-hostable tool, analytics is disabled by default — the <Analytics /> script loads only when ENABLE_ANALYTICS=true is set, so forks and self-hosted deployments ship no tracking script unless they explicitly enable it.

No traffic data is ever sent to the maintainers.

Related Issue

N/A

How to Verify

1. Without ENABLE_ANALYTICS: run npm run build && npm run start, open the app, and confirm no /_vercel/insights script/request appears in the browser Network tab.
2. With ENABLE_ANALYTICS=true: restart and confirm the /_vercel/insights/script.js request now loads.
3. On the demo deployment, also enable Web Analytics in the Vercel dashboard for data to be recorded.

Checklist

• PR title follows Conventional Commits (feat:, fix:, chore:, refactor:, docs:, i18n:)
• Translations added to both locales/ko.json and locales/en.json (if UI text changed)
• Tested on mobile viewport (if UI changed)