Export button in bucket view silently fails: WebView fetches the export but never saves the file

# Export button in bucket view silently fails: WebView fetches the export but never saves the file

## Summary

Tapping **Export** on a bucket in the Android app does nothing (appears to hang / time out). No file is ever produced, no error is shown, and the app does not crash.

Log capture shows the embedded `aw-server-rust` **successfully generates the export in ~39 ms**, but the response is then dropped by the app's WebView so no Android download or Storage-Access-Framework machinery is ever invoked, nothing is written to storage. This looks like the in-app WebView has no `DownloadListener` wired up to handle the web UI's download.

This reproduces on a **fresh install**
## Environment

| | |
|---|---|
| Device | Samsung Galaxy A52s 5G (SM-A528B) |
| Android | 14 (API 34), One UI 6.1 |
| Build | UP1A.231005.007.A528BXXSBGYI3 |
| Security patch | 2025-09-01 |
| ActivityWatch app | **0.12.1b5** (versionCode 33), minSdk 24, targetSdk 34 |
| System WebView | Google Chrome/Android System WebView **150.0.7871.181** |

## Steps to reproduce

1. Open the app, go to a bucket in the buckets view (e.g. `aw-watcher-android-test`).
2. Tap **Export** for that individual bucket.
3. Observe: nothing happens — no file, no picker, no error, no toast.

## Expected behavior

The bucket export JSON is saved to the device (e.g. via a Storage Access Framework "Save to…" picker or the Downloads folder), matching how the desktop web UI download behaves.

## Actual behavior

The web UI issues the export request, the local server answers it successfully, and then the file is silently discarded. From the user's perspective it hangs / times out.

## Evidence (logcat)

The app renders the bundled web UI inside an Android WebView (a Chromium sandboxed process is spawned for it):

```
ActivityManager: Start proc 3758:com.google.android.webview:sandboxed_process0:...:0
for {net.activitywatch.android/org.chromium.content.app.SandboxedProcessService0:0}
```

At the moment Export is tapped, the embedded server handles the request successfully in ~39 ms, then the client (WebView) immediately closes the connection:

```
I aw-server-rust: rocket::server: GET /api/0/buckets/aw-watcher-android-test/export:
I aw-server-rust: rocket::server: Matched: (bucket_export) GET /api/0/buckets//export
I aw-server-rust: rocket::server: Outcome: Success (19:08:28.527 -> 19:08:28.566, ~39 ms)
W aw-server-rust: rocket::server: Remote left: channel closed.
```

Across the entire attempt, **none** of Android's file-saving mechanisms fire (grep over the full capture, 0 hits each):

```
DownloadManager: 0 DownloadListener: 0 ACTION_CREATE_DOCUMENT: 0
documentsui: 0 openOutputStream: 0
```

There is also **no** `FATAL`/`AndroidRuntime` crash, **no** ANR, and **no** `OutOfMemory` in the log. The response is produced and then goes nowhere.

## Likely root cause

The Export action in the bundled `aw-webui` performs a browser-style download (a `blob:` / `` save that works in a normal browser). Android's `WebView` does not handle such downloads unless the host app registers a `WebView.setDownloadListener(...)` (and, for `blob:` URLs, bridges them to a native save). Without that, the fetched bytes are dropped.

## Workaround

Managed to pull the data directly from the phone's local server over ADB, bypassing the WebView:

```bash
# On the phone: enable Wireless debugging (Developer options)
adb pair <6-digit-code>
adb connect
adb forward tcp:5666 tcp:5600
# Per-bucket (reliable):
curl http://localhost:5666/api/0/buckets//export -o .json
```