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/app.js

@@ -70,6 +70,7 @@ async function init() {
   await I18n.init();
   applyStaticI18n();
   setupLanguage();
+  await Pwa.init();
   setupViewerBanner();
   setupNav();
   setupUpload();
@@ -123,6 +124,7 @@ function setupLanguage() {
   sel.addEventListener("change", async (e) => {
     await I18n.setPreference(e.target.value);
     applyStaticI18n();
+    Pwa.refreshHint();
     resetLocaleDependentPanels();
     if (state.data) renderAll();
     const gs = document.getElementById("global-search");