Fix service worker fetch handler and remove duplicate listener

[shiro-all]

Summary

This PR fixes a critical issue in the serviceworker.js:

• Removed shadowing of the global self.
• Eliminated the duplicate fetch event listener to prevent multiple calls to event.respondWith, which was causing uncaught exceptions and breaking the Service Worker.
• Consolidated the fetch handler logic into a single listener:
  • Serves cached responses when available.
  • Falls back to network fetch.
  • If both fail (e.g., offline), serves the offline.html page for navigation requests.

Changes

• Removed const self = this which was incorrectly shadowing the global.
• Rewrote the fetch listener to handle caching/network/offline fallback properly.

Please review and merge. Once deployed, the site should correctly serve the offline page and have a functioning Service Worker without errors.

[github-actions]

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