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/service-worker.js

@@ -0,0 +1,110 @@
+self.addEventListener("fetch", (event) => {
+  const url = new URL(event.request.url);
+  if (event.request.method === "POST" && url.origin === self.location.origin && url.pathname === "/share-target") {
+    event.respondWith(handleShareTarget(event.request));
+  }
+});
+
+const SHARE_CACHE = "warpbox-share-target-v1";
+const SHARE_PREFIX = "/__warpbox_share_target__/";
+const LATEST_KEY = SHARE_PREFIX + "latest";
+
+async function handleShareTarget(request) {
+  const id = Date.now().toString(36) + "-" + Math.random().toString(36).slice(2, 10);
+  try {
+    const formData = await request.formData();
+    const files = collectSharedFiles(formData);
+    const cache = await caches.open(SHARE_CACHE);
+    const metadata = {
+      id,
+      title: stringValue(formData.get("title")),
+      text: stringValue(formData.get("text")),
+      url: stringValue(formData.get("url")),
+      createdAt: new Date().toISOString(),
+      files: [],
+    };
+
+    await deletePreviousShare(cache);
+    for (let index = 0; index < files.length; index += 1) {
+      const file = files[index];
+      const key = SHARE_PREFIX + "file/" + encodeURIComponent(id) + "/" + index;
+      metadata.files.push({
+        key,
+        name: file.name || "shared-file",
+        type: file.type || "application/octet-stream",
+        size: file.size || 0,
+        lastModified: file.lastModified || Date.now(),
+      });
+      await cache.put(key, new Response(file, {
+        headers: {
+          "Content-Type": file.type || "application/octet-stream",
+          "Cache-Control": "no-store",
+        },
+      }));
+    }
+
+    await cache.put(LATEST_KEY, jsonResponse(metadata));
+    await cache.put(SHARE_PREFIX + "meta/" + encodeURIComponent(id), jsonResponse(metadata));
+  } catch (error) {
+    await storeShareError(id, error);
+  }
+
+  return Response.redirect("/?share-target=1&share-id=" + encodeURIComponent(id), 303);
+}
+
+function collectSharedFiles(formData) {
+  const files = [];
+  ["files", "file", "sharex"].forEach((name) => {
+    formData.getAll(name).forEach((value) => {
+      if (value instanceof File && value.size > 0) {
+        files.push(value);
+      }
+    });
+  });
+  return files;
+}
+
+function stringValue(value) {
+  return typeof value === "string" ? value : "";
+}
+
+function jsonResponse(payload) {
+  return new Response(JSON.stringify(payload), {
+    headers: {
+      "Content-Type": "application/json",
+      "Cache-Control": "no-store",
+    },
+  });
+}
+
+async function storeShareError(id, error) {
+  const cache = await caches.open(SHARE_CACHE);
+  await cache.put(LATEST_KEY, jsonResponse({
+    id,
+    error: error && error.message ? error.message : "Shared files could not be staged.",
+    createdAt: new Date().toISOString(),
+    files: [],
+  }));
+}
+
+async function deletePreviousShare(cache) {
+  const previous = await cache.match(LATEST_KEY);
+  if (!previous) {
+    return;
+  }
+  let metadata = null;
+  try {
+    metadata = await previous.json();
+  } catch (error) {
+    metadata = null;
+  }
+  for (const file of metadata && metadata.files ? metadata.files : []) {
+    if (file.key) {
+      await cache.delete(file.key);
+    }
+  }
+  if (metadata && metadata.id) {
+    await cache.delete(SHARE_PREFIX + "meta/" + encodeURIComponent(metadata.id));
+  }
+  await cache.delete(LATEST_KEY);
+}