openai/gpt-chat-latest → A (3:2)

Side A removes the leftover monolithic `server/src/html/forum.rs`, introduces `forum/mod.rs`, and makes the split `forum/` directory the single module root, eliminating duplicate module definitions and reducing long-term maintenance ambiguity without changing behavior. Side B makes a focused behavioral improvement by splitting each unranked sibling into its own navigation group and adds a regression test, but its impact is narrower than the structural cleanup and module-resolution fix in Side A.