Feature: QR code creation wizard (3 steps)

[tom-schorn]

Replace the simple create form with a 3-step wizard.

Steps

1. Basics — name, action type (redirect | vcard | landing), validity
2. Content — per action: redirect URL · pick a vCard · choose a landing template
3. QR design — color, size, error correction, download PNG/SVG

Acceptance

• Stepping through creates a working code with a stable slug
• Step 2 adapts to the chosen action type

Depends on: #5, #6