feat: add visible purchase pricing details

[hubab1]

Summary

• parse the localized visible in-app purchase and subscription rows that Apple exposes on public App Store pages
• add an on-demand disclosure row to each compared app, loading only one expanded app across at most five selected storefronts
• preserve each storefront's native display price, cache results for six hours, cap output at 20 products, and label the public list as potentially incomplete
• provide independent loading, unavailable, retry, truncation, freshness, and accessibility states

Why

The base-price matrix in #70 answers whether an app is free or paid. This follow-up lets users inspect purchase names and localized prices when Apple publicly exposes them, without implying that the list is exhaustive or estimating currency conversion, tax, or developer proceeds.

Validation

• xcodebuild test for RankedAppPricingServiceTests and VisibleProductPricingServiceTests: 11 tests passed
• parser verified against localized public App Store page structures, including native US and German price formatting
• launched the built app and exercised selection, comparison, expansion, loading, and empty-list states

Dependency

This draft is stacked on #70 and indirectly on #69. It should be reviewed and merged after those branches.