Harden PWA installability and offline boot by removing manifest duplication

[Copilot]

EasyTune’s PWA setup had redundant manifest artifacts and a network-only manifest read path that could degrade offline startup metadata. This change consolidates manifest usage and aligns runtime fetch behavior with offline-first expectations.

• Single manifest source of truth

  • Removed manifest.json to eliminate duplicate manifest state.
  • Kept manifest.webmanifest as the only manifest referenced by the app.

• Service Worker cache cleanup

  • Removed ./manifest.json from APP_SHELL in sw.js.
  • Cache preloading now tracks only assets actually used at runtime.

• Offline-safe manifest version read

  • Removed cache: 'no-store' from manifest fetch in runtime version loaders (app.js, js/app-main.js), allowing SW/browser cache to satisfy reads offline.

// before
const res = await fetch('manifest.webmanifest', { cache: 'no-store' });

// after
const res = await fetch('manifest.webmanifest');