Clarify Unraid positioning in app branding

[borg-codex-bot]

Summary

Clarify the app branding in the UI by adding a small Unraid positioning line below the existing Borg Backup UI logo text.

Background

Another BorgBackup web UI project with a similar name exists. The README now positions this project as Unraid-focused; the application header/sidebar should mirror that positioning without renaming the product.

Scope

• Keep the primary UI name as Borg Backup UI.
• Add a small, unobtrusive for Unraid subline below the existing sidebar logo text.
• Keep layout stable in desktop and responsive/sidebar modes.

Acceptance Criteria

• The sidebar branding visually reads as Borg Backup UI for Unraid.
• The navigation layout is not widened or disrupted.
• The change is reflected in the release notes.