Online-only image thumbnails are missing in Windows Explorer

### Self Checks

* [x] I have searched for existing issues, including closed ones.
* [x] I confirm that I am using English to submit this report.
* [x] I have checked that thumbnails are generated correctly in the Cloudreve Web UI.

### Describe the bug

Cloudreve Desktop does not show thumbnails for online-only image files in Windows 11 File Explorer.

The same images show thumbnails correctly in the Cloudreve Web UI.
Also, if I mark the files/folders as "Always keep on this device" and the files are downloaded locally, Windows Explorer thumbnails are displayed correctly.

Therefore, Windows thumbnail generation itself seems to work, and the issue appears to be specific to Cloudreve Desktop's online-only placeholder thumbnail integration.

### Environment

* OS: Windows 11
* Cloudreve Desktop: Microsoft Store / MSIX version
* Cloudreve Desktop package version: 0.2.0.0
* Package name: 2106abslant.Cloudreve_0.2.0.0_x64__sb8s8nw07jjsw
* Connection: local network / no reverse proxy involved
* Cloudreve Web UI thumbnails: working
* Windows Explorer SyncRoot registration: exists
* Local/hydrated file thumbnails in Explorer: working
* Online-only placeholder thumbnails in Explorer: not displayed

### SyncRoot information

The Cloudreve SyncRoot is registered in Windows:

```powershell
HKLM:\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\SyncRootManager

cloudrevefa7b49d121b0786f!S-1-5-21-507098600-2759984182-4059057422-1001!NMiP
```

The registered display name is:

```text
DisplayNameResource: マイファイル
IconResource: C:\Users\miyag\.cloudreve\icos\2962863236fabecd.ico,0
```

### Steps to reproduce

1. Install Cloudreve Desktop from Microsoft Store.
2. Add a Cloudreve account and mount/sync the cloud drive.
3. Open a folder containing image files such as `.jpg` or `.png` in Windows File Explorer.
4. Keep the files in online-only state.
5. Set Explorer view to large icons or extra large icons.
6. Check whether thumbnails are displayed.

### Expected behavior

Windows Explorer should display image thumbnails for online-only files, similar to other cloud file providers such as Nextcloud or OneDrive.

### Actual behavior

Windows Explorer only shows generic icons for online-only image files.

If the same files are downloaded locally using "Always keep on this device", thumbnails appear correctly in Windows Explorer.

### Additional information

* The Cloudreve Web UI displays thumbnails correctly for the same files.
* This is not related to reverse proxy, Cloudflare, Caddy, or authentication, because the test is performed over a local connection.
* This seems to affect only the online-only placeholder thumbnail path in Cloudreve Desktop.
* Explorer integration itself appears to be partially working because the SyncRoot is registered.

### Possible cause

The issue may be related to Cloudreve Desktop's Windows Cloud Files API / Explorer thumbnail provider integration for online-only placeholders.

### Workaround

Marking image folders as "Always keep on this device" makes Explorer thumbnails appear, but this defeats the purpose of online-only files.

Start by reproducing the issue with online-only .jpg or .png files in Windows 11 File Explorer, then inspect the Windows Cloud Files API and Explorer thumbnail integration in Cloudreve Desktop. Done means online-only image placeholders show thumbnails in large and extra-large Explorer views while locally hydrated files continue to work.

Written by the indexing model from the issue text.