Generate app store listing materials (screenshots, descriptions, icons)

Create all materials needed for Google Play, Apple App Store, and Amazon Appstore submissions.

**Need:**
- App icon (512x512 PNG and 1024x1024)
- Feature graphic (1024x500 for Google Play)
- 4-6 screenshots at phone resolution (1080x1920) and tablet (1200x1920)
- Short description (80 chars)
- Full description (4000 chars)
- Keywords
- Category selection (Education > Math)
- Age rating questionnaire answers
- Privacy policy URL

Can generate icon with FLUX.1 and take screenshots with Playwright.

**Acceptance criteria:**
- [ ] All assets in a `release/store-assets/` directory