AdGuard Mail Tracking Protection filter not enabled by default for Privacy protection

### AdGuard version

2.20.0.2402 nightly

### Browser version

Safari 26.5.2 (21624.2.5.11.8)

### OS version

macOS 26.5.2 (25F84)

### Issue Details

Steps to reproduce:
1. Install AdGuard for the first time
2. Go through setup (Setup Assistant)
3. Enable any level of privacy protection

### Expected Behavior

AdGuard Mail Tracking Protection filter is subscribed.

### Actual Behavior

AdGuard Mail Tracking Protection filter is **not** subscribed, and must be manually done afterwards in the Filters tab.

### Additional Information

Also unsure if the AdGuard Mail Tracking Protection filter can be automatically added for existing users who already have "AdGuard Tracking Protection Filter" subscribed, or some other way (like a notification) to let them know that this new filter exists. Made a suggestion to include it as a checkbox in the Stealth Mode tab under Settings (#1735).