layout.tsx pointed <link rel="manifest"> at /manifest.json, but the
actual generated route (app/manifest.ts) serves at
/manifest.webmanifest -- the manifest was silently never picked up.
Its icons also referenced icon-192.png/icon-512.png, which don't
exist (only the .svg versions do). Both together meant Chrome/Android
install eligibility was broken with no visible error.
Also adds apple-mobile-web-app meta tags + viewport-fit=cover for
iOS home-screen installs, and a dismissible install banner driven by
the standard beforeinstallprompt event (Chromium-only by spec).
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>