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/common.js

@@ -58,6 +58,15 @@
   }
 })();
 
+// Registering this is what makes Chrome/Android offer the "Add to Home
+// screen" install prompt -- a manifest link alone isn't enough. Served
+// from /sw.js (not /static/sw.js) so its scope is the whole app.
+if ("serviceWorker" in navigator) {
+  window.addEventListener("load", function () {
+    navigator.serviceWorker.register("/sw.js");
+  });
+}
+
 // Shared display names for widget_type, everywhere one shows up in the
 // UI (Layout canvas, Add-a-widget buttons, button-assignment dropdowns).
 const WIDGET_LABELS = {

server/app/static/manifest.json

@@ -0,0 +1,14 @@
+{
+  "name": "ESPresso Frame",
+  "short_name": "ESPresso",
+  "description": "Manage your e-ink photo frames.",
+  "start_url": "/",
+  "scope": "/",
+  "display": "standalone",
+  "background_color": "#f5f6f8",
+  "theme_color": "#2563eb",
+  "icons": [
+    { "src": "/static/icons/icon-192.png", "sizes": "192x192", "type": "image/png" },
+    { "src": "/static/icons/icon-512.png", "sizes": "512x512", "type": "image/png" }
+  ]
+}

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)));