Add file explorer context menu entry: “Nextcloud → Open with Nextcloud Office”

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## Feature request

**Which Nextcloud Version are you currently using:**
not relevant for this feature request / please fill in your version

**Is your feature request related to a problem? Please describe.**
Yes. Files synced with the Nextcloud Desktop client are usually opened locally with the default desktop application, such as Word, Excel, PowerPoint, LibreOffice, etc.

For collaborative workflows, this is often not what is wanted. In many cases, the user actually wants to open the file directly in **Nextcloud Office** for co-editing. At the moment, this requires several extra steps in the browser: opening Nextcloud, navigating to the file, and opening it there manually.

This makes collaborative editing less convenient, less discoverable, and less integrated into the desktop workflow.

**Describe the solution you'd like**
I would like a context menu entry for synced Office files in the local file explorer, for example:

**Nextcloud → Open with Nextcloud Office**

This should be available for supported Office file types such as:

- `.docx`
- `.xlsx`
- `.pptx`

When selected, the Desktop client should open the corresponding file in the server-side **Nextcloud Office** editor instead of opening it locally in the default desktop application.

Ideally, this should directly lead into the collaborative editing workflow and make it easier to distinguish between:

- opening a file **locally**
- opening a file in **Nextcloud Office for collaboration**

**Describe alternatives you've considered**
At the moment, the alternative is to manually open Nextcloud in the browser, navigate to the file, and then open it in Nextcloud Office from there.

Another alternative is to continue opening the file locally, but that does not help when the goal is collaborative editing.

A further improvement could be opening the file in a dedicated standalone Office window, but that likely belongs more to the Office integration itself. Even without that, the desktop context menu entry alone would already make the workflow much more practical.

**Additional context**
Related issue in `nextcloud/richdocuments`: https://github.com/nextcloud/richdocuments/issues/5482
(Standalone Office window / Office-only PWA for Nextcloud Office)

This request complements that issue from the desktop side: a file explorer entry like **Nextcloud → Open with Nextcloud Office** would be a natural way to launch that standalone collaborative editing workflow directly from synced files.

Typical use case:

- a user has a synced Nextcloud folder on their computer
- they right-click a `.docx`, `.xlsx`, or `.pptx` file
- they choose **Nextcloud → Open with Nextcloud Office**
- the file opens directly in the server-side collaborative editor

This would make the collaborative workflow much easier to discover and use in daily practice.