feat: add PWA support

README.md

@@ -15,6 +15,7 @@ The current MVP slice includes:
 - disabled-by-default, serial-pinned ADB status, APK installation, package inventory, and validated app shortcuts;
 - live MG4 speed, gear, and steering-angle telemetry with a dedicated visual Car dashboard;
 - a responsive 1920×720-oriented setup, login, and dashboard UI;
+- an installable PWA shell with standalone landscape display and cached dashboard assets;
 - an idempotent Raspberry Pi installer with systemd start-on-boot support;
 - atomic, dashboard-triggered Git updates with automatic verification and rollback safety;
 - optional dual-radio Wi-Fi failover with an always-recoverable offline car hotspot;
@@ -106,6 +107,8 @@ npm run dev:web
 
 Open `http://127.0.0.1:5173`. The development server proxies API requests to `http://127.0.0.1:8787`.
 
+The production build includes a web app manifest and service worker. PWA installation and offline shell caching require a secure browser context: HTTPS in production, or `localhost` during local use. The dashboard still requires the Pi connection for live data and authenticated actions.
+
 Local development data is stored in `server/data/companion.db` and ignored by Git.
 
 ## Quality gate