openai/gpt-chat-latest → A (3:2)

Side A introduces a new canonical browse URL scheme (`/~/...`), centralizes link generation with `ItemId::browse_href`, adds parsing helpers and routing for path-based browsing, updates redirects, and includes integration tests, creating a lasting user-facing navigation improvement while removing obsolete theme infrastructure. Side B improves type safety by propagating `CanonicalItemUrl` through APIs and eliminating many string conversions, but it is primarily an internal refactor with limited behavioral change compared with A's new routing and URL model.