feat(ui): add toggle for email subject show level configuration

[ch8matt]

Description

This change introduces a new UI toggle in the Email Configuration page for the
email.subject.show.level setting. This allows administrators to decide whether
notification email subjects should include the notification level (e.g. [ERROR],
[INFORMATIONAL]).

The backend already supports this configuration, and this PR completes the feature
by exposing the toggle in the frontend.

Addressed Issue

DependencyTrack/dependency-track#5503

Additional Details

Checklist

• I have read and understand the contributing guidelines
• This PR introduces new or alters existing behavior, and I have updated the documentation accordingly

[owasp-dt-bot]

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