Add PWA install badge for desktop and tablet (manifest, SW, Add to Home Screen).

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/public/sw.js

@@ -0,0 +1,38 @@
+/* Minimal service worker: makes the app installable on desktop & tablet. */
+const CACHE = "eth-hedge-shell-v1";
+const PRECACHE = ["/", "/plan", "/manifest.webmanifest", "/icons/icon-192.png"];
+
+self.addEventListener("install", (event) => {
+  event.waitUntil(
+    caches.open(CACHE).then((cache) => cache.addAll(PRECACHE)).then(() => self.skipWaiting()),
+  );
+});
+
+self.addEventListener("activate", (event) => {
+  event.waitUntil(
+    caches.keys().then((keys) =>
+      Promise.all(keys.filter((k) => k !== CACHE).map((k) => caches.delete(k))),
+    ).then(() => self.clients.claim()),
+  );
+});
+
+self.addEventListener("fetch", (event) => {
+  const req = event.request;
+  if (req.method !== "GET") return;
+  const url = new URL(req.url);
+  if (url.origin !== self.location.origin) return;
+  // Never cache API / health / websocket-ish paths
+  if (url.pathname.startsWith("/api") || url.pathname === "/health") return;
+
+  event.respondWith(
+    fetch(req)
+      .then((res) => {
+        if (res.ok && (url.pathname.startsWith("/assets") || url.pathname === "/")) {
+          const copy = res.clone();
+          caches.open(CACHE).then((cache) => cache.put(req, copy));
+        }
+        return res;
+      })
+      .catch(() => caches.match(req).then((hit) => hit || caches.match("/"))),
+  );
+});