refactor: Deduplicate chat-related code

[Nicolapps]

Problem: Chat functionality has duplicated patterns between exercise chat (ExplainExercise.tsx + ChatBubble.tsx) and lecture chat. The deprecated MessageInput.tsx (142 lines) still exists alongside the newer PromptInput component that replaced it. Loading dot animations are copy-pasted between ChatBubble.tsx and ExplainExercise.tsx.

Proposed Solution

• Identify and extract shared chat patterns (message list, input, loading indicators) into reusable components
• Remove MessageInput.tsx if fully replaced by PromptInput
• Extract the loading dots animation into a shared <LoadingDots> component