Replace the fragile global DOM query approach in `updateOverallStatus`
with per-batch closure-based counters inside `processFiles`.
- Add `total`, `done`, `updateStatus()`, and `markDone()` as closure
variables/functions within each `processFiles` invocation
- Change `_uploadSingleFile` to accept an `onTerminal` callback instead
of `statusMessage`, called when a file reaches a terminal state
- Pass `markDone` as the `onTerminal` callback from `scheduleNext`
- Remove the now-unused global `updateOverallStatus` function
The previous implementation queried `document.querySelectorAll('.file-status')`
globally and relied on text `startsWith` checks to count completed files.
This was fragile and could produce a stale done=0 count in practice.
The new approach uses deterministic closure counters, so the displayed
"Uploading files (X/N)" count correctly increments as files complete.
Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>
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