Architecture hardening phase 3: decompose migration wizard steps

## Problem / need

`migration-wizard.component.ts` is ~2100 LOC with inline template and styles. Hard to review and extend.

## Proposed solution

**Task 3.3** (this issue):
- Extract step components: products, strategy, review (+ step nav)
- Move shared wizard styles to external SCSS
- Container keeps orchestration, API calls, and state
- `npm test` must pass

## Impacted repos

- [x] gateforge