Add setting to persist Screencast Mode (settings.json)

Screencast Mode is currently a toggle-only UI state (`Toggle Screencast Mode`) and cannot be configured via `settings.json`.

For users who switch between VS Code Profiles, this creates friction:
- In a teaching/recording profile, Screencast Mode should always be on.
- In a daily coding profile, it should remain off.
- Today this must be toggled manually after profile switches or restarts.

## Proposal
Expose Screencast Mode as a configurable setting that can be stored per profile, for example:

```json
"workbench.screencastMode.enabled": true
```
## Expected behavior

1. If enabled in Profile A, Screencast Mode is active whenever Profile A is used.
2. If disabled in Profile B, it stays off in Profile B.
3. State is preserved across restarts and profile switches.

## Why this helps

- Better profile isolation
- Less repetitive manual toggling
- More reliable setup for demos, classes, and recordings