UI: Branch picker when advancing a purchase process

[b-at-neu]

Part of #530
Blocked by #595, #596

When completing a step that has multiple children, show a picker so the user can choose which branch to follow next.

Changes

• Detect when the current step has >1 children
• Show a selection UI: 'Choose next step: [Option A] or [Option B]'
• Auto-advance when only one child exists (preserves current linear behavior)
• Update currentStepId on PurchaseProcess to the chosen branch