Add two-user taste comparison experience

[ALeonard9]

Summary

• add a responsive comparison page for friends and visible public profiles
• compare all four domains together or filter to one shelf
• show shared queues, alignment, biggest gaps, closest agreement, and top unseen recommendations
• provide consistent Watchlist and Rank actions with tracked-state highlighting
• explain private or unavailable profiles and offer a friend-request path where appropriate
• show recommendation provenance on watchlist cards

Verification

• full web suite: 200 passed
• production build passed
• ESLint and TypeScript passed
• changed-test pre-push suite passed
• visually verified at desktop and PWA widths

Depends on ALeonard9/druthers-api#304.

Closes #126