Make the server installable as a home-screen PWA

Adds a web manifest, hand-drawn cup+frame icons, and a presence-only
service worker (no offline caching) so mobile browsers offer
"Add to Home Screen" for the server UI.

server/app/static/sw.js

@@ -0,0 +1,8 @@
+// Presence-only service worker: satisfies the "installable" requirement
+// (Chrome/Android in particular checks for a controlling SW with a fetch
+// handler) without adding an offline cache -- every request just goes to
+// the network as normal. Served from / (see app/main.py's /sw.js route)
+// so its scope covers the whole app, not just /static/.
+self.addEventListener("install", () => self.skipWaiting());
+self.addEventListener("activate", (event) => event.waitUntil(self.clients.claim()));
+self.addEventListener("fetch", (event) => event.respondWith(fetch(event.request)));