feat(backend): handle processing errors and add PWA routes

- Block file downloads and previews with a 424 StatusFailedDependency if file processing failed or the box has issues.
- Register routes for `/service-worker.js` and `/share-target` to support PWA features.
- Update README.md with an AI usage disclosure.

backend/static/js/02-pwa.js

@@ -0,0 +1,43 @@
+(function () {
+  let installPrompt = null;
+
+  if ("serviceWorker" in navigator) {
+    window.addEventListener("load", () => {
+      navigator.serviceWorker.register("/service-worker.js").catch(() => {
+        /* Service workers are progressive enhancement here. */
+      });
+    });
+  }
+
+  window.addEventListener("beforeinstallprompt", (event) => {
+    const button = document.querySelector("[data-install-pwa]");
+    if (!button) {
+      return;
+    }
+    event.preventDefault();
+    installPrompt = event;
+    button.hidden = false;
+    button.addEventListener("click", async () => {
+      if (!installPrompt) {
+        return;
+      }
+      button.disabled = true;
+      try {
+        await installPrompt.prompt();
+        await installPrompt.userChoice;
+      } finally {
+        installPrompt = null;
+        button.hidden = true;
+        button.disabled = false;
+      }
+    }, { once: true });
+  });
+
+  window.addEventListener("appinstalled", () => {
+    const button = document.querySelector("[data-install-pwa]");
+    if (button) {
+      button.hidden = true;
+    }
+    installPrompt = null;
+  });
+})();