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/templates/pages/home.html

@@ -10,7 +10,7 @@
     {{end}}
   </div>
 
-  <form class="upload-grid" id="upload-form" action="/api/v1/upload" method="post" enctype="multipart/form-data">
+  <form class="upload-grid" id="upload-form" action="/api/v1/upload" method="post" enctype="multipart/form-data" data-max-upload-bytes="{{.Data.MaxUploadBytes}}" data-max-upload-label="{{.Data.MaxUploadSize}}">
     <div class="card upload-main">
       <div class="card-content">
         {{if .CurrentUser}}
@@ -76,6 +76,7 @@
 
         <div class="form-footer">
           <p id="file-summary">Choose one or more files to begin.</p>
+          <button class="button button-outline install-pwa-button" type="button" data-install-pwa hidden>Install Warpbox</button>
           <button class="button button-primary" type="submit">Upload files</button>
           <button class="button button-danger upload-new-button" type="button" id="new-upload" hidden>New upload</button>
         </div>