Restructure ViewModel architecture with base class and UI layer conventions

[cynthiabondi]

Related PRs: #1204

Summary
Introduce a shared ViewModel base class and document UI layer conventions to standardize how ViewModels interact with the UI across the codebase.

Motivation
ViewModel implementations are inconsistent, large when branches, no shared structure, and no documented conventions make the codebase harder to maintain and onboard contributors.

Acceptance criteria

• Create a ViewModel base class with shared behavior and rules
• Extract large when branches to private functions across existing ViewModels
• Document ViewModel/UI layer conventions in the architecture doc
• All CI checks pass after restructuring

References
Closes #1204

[cynthiabondi]

Completed in #1204