[UX] "Fork from here" button placement — should be under Claude responses, not user prompts

The "Fork from here" button is currently located below user prompts, which doesn't match the logical flow of the UI. When reviewing a conversation, the fork action is most relevant to Claude's response above it—not the user's prompt below it. The button should be repositioned beneath Claude responses, directly next to the "Pin chapter" button at the bottom of each response block.

Proposed Solution

Move the "Fork from here" button to appear beneath each Claude response, positioned directly next to the "Pin chapter" button. This groups the fork action with the response it acts upon, creating a more intuitive mental model where both actions are associated with the message they apply to.