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/libs/handlers/upload.go

@@ -53,6 +53,11 @@ func (a *App) Upload(w http.ResponseWriter, r *http.Request) {
 	}
 	if err := r.ParseMultipartForm(parseLimit); err != nil {
 		a.logger.Warn("upload form parse failed", withRequestLogAttrs(r, "source", "user-upload", "severity", "warn", "code", 4000, "user_id", user.ID, "error", err.Error())...)
+		var maxBytesErr *http.MaxBytesError
+		if errors.As(err, &maxBytesErr) {
+			helpers.WriteJSONError(w, http.StatusRequestEntityTooLarge, "upload exceeds the configured upload limit")
+			return
+		}
 		helpers.WriteJSONError(w, http.StatusBadRequest, "upload form could not be read")
 		return
 	}
@@ -244,7 +249,7 @@ func (a *App) checkUploadPolicyForSizes(r *http.Request, user services.User, log
 		maxBytes := services.MegabytesToBytes(policy.MaxUploadMB)
 		for _, fileSize := range fileSizes {
 			if fileSize > maxBytes {
-				return http.StatusRequestEntityTooLarge, "file exceeds upload size limit"
+				return http.StatusRequestEntityTooLarge, "file exceeds upload size limit of " + services.FormatMegabytesLabel(policy.MaxUploadMB)
 			}
 		}
 	}