Use relative asset URLs, add maskable icon and offline handling in service worker

[Ivanx32]

Motivation

• Ensure assets resolve correctly when the app is served from different base paths and make the PWA manifest more complete by adding background_color and a maskable icon entry.
• Improve offline behavior by consistently referencing the precached offline page and handling the SKIP_WAITING message shape from the service worker registration.

Description

• Converted absolute paths to relative paths in app/index.html for manifest, icon, and the app script by changing /... to ./... and removed a placeholder comment.
• Made the app header use the runtime base URL for the logo by constructing iconPath with import.meta.env.BASE_URL and replaced the hardcoded /icons/placeholder.svg with that variable in app/src/App.tsx.
• Updated the service worker in app/src/sw.ts to compute OFFLINE_URL relative to the service worker scope, accept message objects with { type: 'SKIP_WAITING' }, and use OFFLINE_URL in precacheAndRoute and when falling back to the cached offline page.
• Enhanced the PWA manifest in app/public/manifest.json and vite.config.ts by adding background_color and a maskable icon entry, and changed icon src entries to relative paths (removed leading /).

Testing

• Ran a TypeScript type check with tsc --noEmit and it succeeded.
• Performed a Vite production build with npm run build and the build completed successfully with the service worker precache including the offline page.
• Executed the test suite with npm test and all automated tests passed.

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Codex Task