A regenerate response button is present

AI responses are non-deterministic and fail in soft ways all the time — hallucinations, cut-off streams, refusals on benign prompts, partial answers that drift off topic. Without a regenerate button the only recovery is to retype the prompt verbatim, which users will not do; they will instead give up on the answer or, worse, act on the bad one. Regenerate is the standard recovery affordance established by every reference chat product in the category.

In src/components/chat/MessageBubble.tsx, render a regenerate button on the last assistant message wired to reload() from useChat (or a custom resubmit of the last user message). In error states, promote it to the primary recovery action with a "Try again" label.

{isLastAssistantMessage && !isLoading && (
  <Button onClick={reload} variant="ghost" size="sm">
    <RotateCcw className="h-4 w-4 mr-1" /> Regenerate
  </Button>
)}