Open synced Office files in native desktop editor via document server on double-click

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## Feature request
Open synced Office files in native desktop editor via document server on double-click

When a user double-clicks a ".docx", ".xlsx" or ".pptx" file inside the local Nextcloud sync folder, the file is currently opened as a normal local file by the default desktop application.

## Desired behavior:

Double-clicking a supported Office file in the Nextcloud sync folder should open it in the configured native desktop office editor through the Nextcloud Office / document server session, not as a detached local copy.

## Expected flow:

"Windows Explorer / Finder / file manager → double-click synced Office file → Nextcloud Desktop detects cloud file → opens server-backed editing session → native desktop editor opens document with co-authoring, locking and versioning"

# This would provide Microsoft 365 / SharePoint-like behavior:

- users keep the normal file-manager workflow
- native desktop editor is used
- real-time co-authoring remains available
- document-server locking is respected
- sync conflicts are avoided

A context-menu action like “Open with Nextcloud Office” is useful, but it should not be the main workflow. The normal double-click should be cloud-aware for supported Office documents.

## Main goal:

Make synced Nextcloud Office documents behave like SharePoint/OneDrive + Microsoft 365: double-click from the local sync folder, edit in the desktop app, but still work through the server-backed collaborative document session.

No files, tests, or entry points are identified in the issue, so first map where the desktop client handles double-clicks and synced Office documents. Determine how a document-server session could be selected for supported .docx, .xlsx, and .pptx files without inventing a local-copy workflow. Done means double-clicking those files opens the native editor through the server-backed session while preserving co-authoring, locking, versioning, and conflict avoidance.

Written by the indexing model from the issue text.