refactor: migrate Next.js to Vite SPA, deploy on Cloudflare Workers

The app is a pure client-side SPA (static JSON via fetch, localStorage state,
client clock/routing) with no SSR usage. Next.js App Router + static export
was fighting the grain and broke CI; move to a lightweight static SPA served
by Cloudflare Workers static assets.

- Stack: Vite + React + @tanstack/react-router + @tanstack/react-query
- styled-components retained; PWA via vite-plugin-pwa; jest -> vitest
- Drop unmaintained use-seconds (react17 peer) for a local clock hook
- pnpm (Node 22); static data moved to public/static
- Deploy: Cloudflare Workers static assets (wrangler.toml [assets],
  not_found_handling=single-page-application); push-triggered via Workers Builds
- Remove Next.js / Netlify / storybook config and dependencies

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: elzup

.babelrc

@@ -1,4 +1,9 @@
 node_modules
-.next
+dist
+*.local
 
-public/
+# vite-plugin-pwa generated output (built into dist/)
+dev-dist
+
+# wrangler local cache
+.wrangler

.eslintrc

@@ -1 +1 @@
-14.19.0
+22