Add minimal installable PWA support

- serve manifest and service worker from the app root for install compatibility
- add manifest metadata, service worker registration, and Apple touch icon links to the base template
- add install icon assets for Android, iOS, and desktop install flows
- document deployment and validation notes for the new PWA support
- cover the new endpoints and template output with tests

app/static/service-worker.js

@@ -0,0 +1,7 @@
+self.addEventListener("install", function () {
+    self.skipWaiting();
+});
+
+self.addEventListener("activate", function (event) {
+    event.waitUntil(self.clients.claim());
+});