DISPLAY_MANIFEST previously gated the entire PWA manifest link and
theme-color meta, so white-label installs that set the flag to false
to hide Chatwoot branding also lost the mobile "Install app" prompt.
Serve /manifest.json from a new dynamic controller that reflects
INSTALLATION_NAME, LOGO_THUMBNAIL and a new BRAND_COLOR config, and
keep the manifest link, theme-color and apple-mobile-web-app meta
emitted regardless of DISPLAY_MANIFEST. The flag now gates only the
Chatwoot-branded raster assets (favicons, apple-icon PNGs, ms-icon,
marketing meta description).
