fix PWA asset paths

[gergelyk]

I noticed that app shell isn't correctly cached by the service worker. Consequently PWA doesn't start correctly.

Steps to reproduce

• Build and serve the app: trunk serve
• Open app in the browser (chromium)
• Click "Install egui Template PWA"
• Close PWA
• Stop the server (trunk)
• Start previously installed PWA

Proposed solution

1. trunkrs adds hashes to the file names: eframe_template-*. This makes paths in sw.js under filesToCache invalid. I solved it by adding filehash = false. The other option could be generating sw.js with correct filenames in post_build hook of trunkrs.
2. I noticed that when manifest.json is served from assets/, then installed PWA starts from attempting to read assets/index.html from cache. The file is non-existing, so startup fails. I solved it by moving manifest.json to the root directory. This required relative paths in manifest.json to be updated.