[Feature Request]: Schema Designer Offline Option / Create ERD

### Feature Description

Add an option to the schema designer to work in offline mode, saving the design to a file which can be checked into Git, this would allow large databases to have many views depending on task being performed.

The relationships created within the designer can then be used to enhance IntelliSense/Copilot Suggestions.

### Problem and Motivation

It would be great to use Schema Designer to document the relationships in my database **without** enforcing foreign keys within MSSQL.

Currently opening Schema Designer drops all tables in the database into a tiny view, it would be better to start with a blank canvas and drag in the tables we want, dragging relationships would then build knowledge in the background so if the same tables were dropped in again the designer would know how they are linked, then allow the users to save these views for documentation within their projects.

### Related Area

- [ ] Connection dialog (SQL Server | Azure browse/Fabric browse)
- [ ] Query results panel
- [ ] Query editor
- [ ] Object Explorer
- [ ] Table Designer
- [ ] Schema Compare
- [x] Schema Designer
- [ ] Local SQL Server Container provisioning
- [ ] SQL database in Fabric provisioning
- [x] GitHub Copilot integration
- [ ] Query Plan Visualizer
- [ ] Other (please describe below)

### If you selected "Other", please describe the area

_No response_

### Confirmation

- [x] I have searched existing feature requests and couldn't find a match
- [ ] I want to help implement this feature

No files or tests are named. Start by locating the Schema Designer entry point and the GitHub Copilot integration, then trace how tables and relationships are currently represented. Done should cover a blank, selectively populated canvas, saved offline views that can be checked into Git, and relationship data available to suggestions.

Written by the indexing model from the issue text.