Add PWA manifest, service worker, and install hint.

Enables home-screen installation with per-viewer scope and platform-specific guidance in EN/DE.

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

static/sw.js

@@ -0,0 +1,38 @@
+const CACHE = "if-viewer-static-v1";
+const ASSETS = [
+  "/static/style.css",
+  "/static/favicon.svg",
+  "/static/icon-192.png",
+  "/static/icon-512.png",
+  "/static/i18n.js",
+  "/static/locales/en.json",
+  "/static/locales/de.json",
+];
+
+self.addEventListener("install", (event) => {
+  event.waitUntil(
+    caches.open(CACHE)
+      .then((cache) => cache.addAll(ASSETS))
+      .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) => {
+  if (event.request.method !== "GET") return;
+
+  const url = new URL(event.request.url);
+  if (url.pathname.includes("/api/")) return;
+  if (!url.pathname.startsWith("/static/")) return;
+
+  event.respondWith(
+    caches.match(event.request).then((cached) => cached || fetch(event.request))
+  );
+});