Unable to download all buckets in json format for export.

- [x] I am on the [latest](https://github.com/ActivityWatch/activitywatch/releases/latest) ActivityWatch version available for Android through Google Play: `0.14.0b2`.
- [x] I have searched the issues of this repo and believe that this is not an exact duplicate.

Related Android issue: [ActivityWatch/aw-android#143 - "Exporting and server not working"](https://github.com/ActivityWatch/aw-android/issues/143). This report adds current-production-version reproduction details and a specific observed failure point.

- **OS name and version**: Android 17 (Google Pixel 9)
- **ActivityWatch version**: Android app `0.14.0b2` (current Google Play production release); Web UI backend `v0.14.0 (rust)`
- **Android package**: `net.activitywatch.android`

## Describe the bug

On the current Google Play release, choosing **Export all buckets as JSON** appears to generate the complete export but never gives the user a way to save it. No Android Save/Create Document picker opens, Firefox/the download manager receives no download, and no file is saved. The application remains responsive and shows no error.

This is especially notable because the Google Play release notes for `0.14.0b2` say: `fix: save bucket exports from the WebView instead of dropping them (#229)`. The save handoff still fails in this case.

## To Reproduce

1. Install/update ActivityWatch from Google Play; installed version in this report is `0.14.0b2`.
2. Use a populated ActivityWatch database. In this case, the full JSON export is approximately 57 MB / 57,182,325 characters.
3. Open ActivityWatch and navigate to **Buckets**.
4. Choose **Export all buckets as JSON**.
5. Observe that neither a browser/download-manager download nor an Android Save/Create Document picker appears.
6. Wait several minutes; there is still no saved file or user-visible error.

Optional diagnostic reproduction:

```text
adb logcat "WebUI:V" "*:S"
```

After triggering the export, the following is logged:

```text
I WebUI: Export save requested: aw-bucket-export.json (application/json, 57182325 chars)
```

## Expected behavior

Once the JSON export has been generated, the user should be able to save it. Either of these outcomes would be acceptable:

- The export is delivered to the browser/download manager, as it was previously; or
- An Android Save/Create Document picker opens and the user can choose a destination for the JSON file.

## Documentation

### Observed result

- No file is downloaded or saved.
- No Android Save/Create Document picker appears.
- Firefox, the default browser, receives no download.
- No user-visible error, AndroidRuntime exception, or crash occurs.
- The app remains responsive.
- No additional relevant `WebUI` log line appears after the 57,182,325-character "Export save requested" message.

### Previous working behavior

Before the recent update, the same operation on the same device/data completed as a normal Firefox download and the JSON file was saved successfully.

### Structured diagnostic observations

These observations are factual results of read-only testing; they are not a claim of root cause.

1. **Export generation completes.** The Android-side log records a complete `57,182,325`-character payload for `aw-bucket-export.json`.
2. **The failure is after that logged handoff.** After the message, there is no visible destination UI, download, saved file, error, or crash, even after waiting several minutes.
3. **Payload size may be relevant.** The failed export is large (about 57 MB). Since the complete payload is logged as received, any size-related problem appears to occur after generation and transfer rather than during JSON generation.
4. **The local API is not a practical recovery route in this build.** The local `/api/0/export` endpoint requires Bearer authentication, while the live UI did not expose API Authentication, an API key, or an "Open in browser" action.

## Additional context

This may be related to [aw-android#143](https://github.com/ActivityWatch/aw-android/issues/143), which reports failed JSON bucket export and inability to use the server from a browser. The current report is not intended to duplicate its general symptom; it provides a reproducible result from the current Google Play production release and shows that the full export payload reaches the Android export/save path before the user-facing save step fails to appear.

API-key/browser-access work has also been tracked in [aw-android#145](https://github.com/ActivityWatch/aw-android/issues/145), which was closed by `#156`. The missing live UI controls are included here because they remove a potential authenticated-API workaround for the export failure, not as a proposed diagnosis of the export defect.

Diagnostics were performed without modifying ActivityWatch data. This report intentionally omits device serial numbers, API keys, account information, event contents, bucket contents, and other private data.

## Documentation

## Additional context