Implement UI for NethVoice Guided Installation Flow

[gsanchietti]

Brief description
Implement the UI for the NethVoice guided installation flow as designed in issue #7659. The goal is to provide admins with a clear, step-based installation experience for NethVoice and Proxy NethVoice, reducing confusion and support tickets.

Proposed solution
Develop the multi-step modal UI and status/cards as described in #7659, following the UX/design spec. The implementation should:

• Guide the admin through Account Provider selection/creation, Proxy installation/configuration, and NethVoice app setup
• Include all conditional steps and validations (domain, password, etc.)
• Show conditional interstitials and loading states
• Display status and services cards post-install
• Follow navigation, error, and loading interaction rules
• Use Figma mockups as reference: Figma link

Acceptance tests

• The multi-step modal displays all required steps in the correct order, including conditional ones
• Validation errors appear under relevant fields and at step level as designed
• Navigation (Next/Previous) works, preserving entered data
• Proxy install interstitial and loading overlays behave as specified
• Service cards and status tab display correct information post-setup
• All screens and components match the Figma mockups
• All interaction rules (error, loading, navigation) are correctly implemented

Additional context
See issue #7659 for complete UX spec.
See Figma mockups for component layout and styles: Figma link

See also

• Improve NethVoice installation flow #7659 Improve NethVoice installation flow
• Figma: https://figma.com/design/OsYqPXdkKJz0tBJRsiQgAf/NethServer-shared-file?node-id=2935-13668&p=f&t=v6LNNNT6A7XXs6BJ-0