feat: add permission toggle for public sharing of notes

This commit introduces a new permission toggle that allows administrators to control whether users can publicly share their notes.

- Adds a new environment variable `USER_PERMISSIONS_NOTES_ALLOW_PUBLIC_SHARING` to control the default setting.
- Adds a `public_notes` permission to the `sharing` section of the user permissions.
- Adds a toggle switch to the admin panel for managing this permission.
- Implements backend logic to enforce the permission when a user attempts to share a note publicly.

backend/open_webui/routers/users.py

@@ -157,6 +157,7 @@ class SharingPermissions(BaseModel):
     public_knowledge: bool = True
     public_prompts: bool = True
     public_tools: bool = True
+    public_notes: bool = True
 
 
 class ChatPermissions(BaseModel):