Add offline-first PWA functionality with automatic update detection

[Copilot]

This PR implements offline-first Progressive Web App (PWA) functionality for Coil Timer, enabling the app to work without internet connection after initial load and providing automatic update notifications.

Key Features

Offline-First Functionality:

• Service worker automatically caches all static assets (JS, CSS, HTML, icons, fonts)
• App works completely offline after first visit/installation
• Shows "Ready for Offline Use" notification when offline capability is enabled
• Cache-first strategy ensures fast performance even when online

Automatic Update Detection:

• Git commit hash injected into HTML meta tag during build process
• Service worker checks for updates every 10 minutes
• When new version detected, shows blue "Update Available" notification
• Users can update immediately or dismiss the notification
• Seamless update experience with automatic page reload

Technical Implementation

• vite-plugin-pwa: Automatic service worker generation with minimal configuration
• Workbox: Robust caching strategies and update mechanisms
• Git-based versioning: Reliable version tracking using commit hashes
• TypeScript support: Proper types for PWA virtual modules
• GitHub Pages compatible: Works with existing deployment pipeline

User Experience

Before: App shows "offline" message when no internet connection is available.

After:

1. First visit automatically enables offline functionality
2. Subsequent visits work instantly without internet
3. Timer functions (start/stop/adjust) work offline
4. Update notifications appear when new versions are deployed
5. One-click updates with immediate availability

The implementation uses industry-standard PWA patterns and integrates seamlessly with the existing Vite build process and GitHub Pages deployment.

Fixes #14.

Warning

Firewall rules blocked me from connecting to one or more addresses

I tried to connect to the following addresses, but was blocked by firewall rules:

• esm.ubuntu.com
  • Triggering command: /usr/lib/apt/methods/https (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

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