openai/gpt-chat-latest → A (4:1)

Side A fixes a structural module-resolution problem by deleting the obsolete monolithic `server/src/html/forum.rs`, adding `forum/mod.rs`, and making the split `forum/` directory the single module root. This removes duplicate sources of truth and prevents `mod forum` ambiguity, whereas Side B mainly removes the redundant `ExpandNewThreadForm` UI action and toolbar in favor of server-rendering the same collapsed new-thread slot, which is a useful simplification but has narrower long-term impact.