feat: upgrade to Next 15 / React 19 and overhaul countdown reliability

[ififi2017]

Framework & tooling:

• Upgrade Next.js 14 → 15.5 and React 18 → 19 (async route params)
• Replace unmaintained next-pwa with @serwist/next; migrate runtime caching 1:1 into app/sw.ts (static CacheFirst, locales SWR, pages/API NetworkFirst)
• Switch to ESLint 9 flat config; drop legacy .eslintrc.json
• Resolve all npm audit vulnerabilities (24 incl. 1 critical → 0)
• Add Vitest unit tests and GitHub Actions CI (lint + test + build)

Bug fixes:

• Stop the 15-minute reminder from firing ~60 notifications; guard Notification API availability and permission before use
• Fix overnight shifts (e.g. 22:00-06:00) breaking after midnight
• Replace unreliable Date-string parsing with setHours()
• Replace alert() dialogs with inline, accessible error messages

Security:

• Add locale allow-list validation in server i18n and manifest routes to prevent path traversal
• Wrap manifest Referer parsing in try/catch

PWA / caching:

• Fix runtime cache rules previously hardcoded to an old domain
• Drop cache-busting timestamp on locale fetches so translations work offline
• Remove duplicate service-worker registration; untrack generated sw.js

Features:

• Add configurable "working days per month" for monthly salary, with translations for all 19 locales

Chore:

• Extract countdown/salary logic into lib/countdown.ts
• Remove unused dependencies (rimraf, i18next-http-backend, etc.)
• Drop viewport userScalable:false for accessibility

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Project	Deployment	Actions	Updated (UTC)
off-work-30	Ready	Preview, Comment	Jul 5, 2026 1:05pm