fix(desktop): DuplicateEntryError in add_workspace_to_desktop () and unresponsive success toast

Information about bug

Bug Description

There is a visual and system bug when adding a Workspace Shortcut to the desktop using the "Add to Desktop" functionality in Frappe V16. It raises a DuplicateEntryError and the success notification is broken.

1. The DuplicateEntryError Crash

When calling frappe.desk.doctype.desktop_icon.desktop_icon.add_workspace_to_desktop(workspace), the function unconditionally executes frappe.new_doc() on both Workspace Sidebar and Desktop Icon models without guarding for pre-existing documents.

Since Desktop Icon relies on autoname: "field:label" (which must be unique), attempting to insert a secondary workspace shortcut that shares a common label (such as those already present during core installations) violently crashes.

**Code impacting this: (desktop_icon.py)**
```python
@frappe.whitelist()
def add_workspace_to_desktop(workspace: str):
    sidebar = frappe.new_doc("Workspace Sidebar")  # Blind creation
    ...
    sidebar.save()

    new_icon = frappe.new_doc("Desktop Icon")  # Blind creation
    ...
    new_icon.insert()  # Crash here on duplicates
    return {"icon": new_icon.as_dict()}

####2. The Broken Success Toast Trigger In workspace.js, the standard .call() back checks r.message.status to render the success toast indicating "Workspace added to desktop". However, the API returns {"icon": {...}} instead of .status. Because of this mismatch, the notice is never fired, displaying nothing to the user even if they are fortunate enough to avoid the duplicate error. Suggested Fixes Fix frappe/erpnext#1: Add document guards to desktop_icon.py Guard insertions with existence checks to reuse existing items:

@frappe.whitelist()
def add_workspace_to_desktop(workspace: str):
    if frappe.db.exists("Workspace Sidebar", workspace):
        sidebar = frappe.get_doc("Workspace Sidebar", workspace)
    else:
        sidebar = frappe.new_doc("Workspace Sidebar")
        sidebar.title = workspace