Landing page: per-style IAP section (Jazz Waltz, Bossa Nova, Swing 5/4)

## Summary

Add **per-style IAP** presentation to the JazzJam landing page — showcasing the individual style purchases (Jazz Waltz, Bossa Nova, Swing 5/4) available alongside the Pro Unlock.

## Business Value

Per-style IAPs (monetisation Phase 4) monetise the style catalogue for users who want more musical variety without a subscription. The landing page should surface that the app's style library is expandable — communicating variety drives both installs and post-install purchase intent.

## Scope

### In scope
- A landing-page section presenting the purchasable styles (Jazz Waltz, Bossa Nova, Swing 5/4)
- Per-style short description and pricing
- Consistent visual language with the Pro Unlock section
- Responsive layout (desktop + tablet + mobile)

### Out of scope
- Pro Unlock section — separate issue
- Subscription tier presentation — separate issue
- In-app style menu lock states / per-style paywall (app repo)

## Acceptance Criteria

- [ ] Landing page has a per-style IAP section listing Jazz Waltz, Bossa Nova, and Swing 5/4
- [ ] Each style has a short description and price (placeholder copy acceptable until pricing is settled)
- [ ] Visual language is consistent with the Pro Unlock section
- [ ] Section renders correctly on desktop, tablet, and mobile

## Manual Verification

1. Open the landing page — confirm the per-style IAP section lists all three styles with descriptions and prices
2. Confirm the styles can be added incrementally as each releases (Jazz Waltz first)
3. Resize to tablet and mobile — confirm the section reflows without overflow or clipping