More reliable uploads for large batches

We rebuilt parts of the upload flow to handle large batches of files.

• File checks no longer hang, and your files show in the list as soon as you add them.
• The upload drawer lists every queued file, no matter how many you add.
• "Cancel all" now stops every upload, and asks you to confirm first.
• Your browser warns you before you refresh or leave the page during an upload.
• Large batches of photos upload more steadily.
• When you delete many files at once, the list updates right away.