Prevent photo loss on page reload mid-upload

The browser now warns you before leaving the page while photos are uploading — no more silent data loss from tab close or reload.

Bug Fixes

Photo uploads were silently cancelled if you closed the tab, reloaded the page, or navigated away mid-batch—only discovered when a 287-photo upload stopped at ~200 with no warning. Now the browser shows its standard "changes you made may not be saved" confirmation while any upload is in flight, so you know to wait or intentionally complete the batch before leaving.