openai/gpt-chat-latest → A (3:2)

Side A introduces a new canonical browse URL scheme (`/~/...`) with `ItemId::browse_href`, parsing helpers, routing, and integration tests, replacing ad hoc query-string links and centralizing URL generation. Although it also removes theme support, the core change establishes a cleaner navigation design reused across the UI. Side B mainly refactors existing web post/check/redact flows into the `/ui` dispatcher and consolidates session handling, improving structure but with less new end-user capability.