Restructure ViewModel architecture with base class and UI layer conventions

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