Adds vite-plugin-pwa with a generated manifest, icon set derived from
the Stalwart mark (192/512/maskable/apple-touch), and theme-color meta
tags. /api and /jmap are excluded from the service worker's navigate
fallback and are never precached or runtime-cached, so authenticated
mail data can't end up in Cache Storage.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
