Audit & optimise generated templates for all supported architectures

[Arjun544]

Review the Handlebars templates in templates/flutter/ for each supported
architecture (Clean Architecture, MVVM, Feature-First) and ensure the
generated Dart output follows best practices for that pattern.

Checklist:

• Clean Architecture - verify layer separation (data/domain/presentation),
  no cross-layer imports in generated code
• MVVM - verify ViewModel doesn't bleed UI logic, bindings are clean
• Feature-First - verify each feature folder is fully self-contained
• Run Dart analyze on the generated output for each architecture with zero errors
• Test all flag combinations (state management, routing, theme) per architecture

References:

• templates/flutter/
• docs/template-development.md
• docs/architecture.md