[GREEN] Implement desktop UX/UI validation coverage and acceptance checks

[ksamaschke]

Phase

GREEN

## Deliverable
Implement the minimum validation coverage needed to satisfy the desktop acceptance tests and checks.

## Dependencies
- Parent work item: #220.

## Completion evidence
- Validation implementation notes or coverage references.

• Evidence that the RED validation checks now pass.

  Acceptance criteria

  • The validation suite checks the redesigned desktop shell, Settings window, and themed variants.

• The checks enforce visible UX/UI completion rather than infrastructure completion.

  Parent: Work Item: Add screenshot-level validation and desktop acceptance checks #220