[FEATURE] Add architecture preset selection to interactive init

[bnbong]

Summary

Add an Architecture Preset step to fastkit init --interactive so users choose a project structure before selecting stack features.

Scope

• Add a new prompt early in the interactive flow
• Supported presets for v1.3.0:
  • minimal
  • single-module
  • classic-layered
  • domain-starter
• Show a short description for each preset
• Store the selected preset in the interactive config
• Show the preset in the final confirmation summary
• Choose and document a default selection for the new step

Out of Scope

• Implementing the full generation behavior for each preset in this issue
• Refactoring unrelated interactive prompts

Acceptance Criteria

• Users see a dedicated preset selection step in interactive mode
• The selected preset is preserved in the final config
• The selection appears in the summary table before confirmation
• Docs/help text reflect the new step
• The current interactive flow remains usable even before preset-specific generation is fully extended

Suggested Areas

• src/fastapi_fastkit/backend/interactive/prompts.py
• src/fastapi_fastkit/backend/interactive/config_builder.py
• src/fastapi_fastkit/backend/interactive/selectors.py
• Interactive CLI tests