Fix chat input lag in long chats

[mcintyre94]

Summary

• Replace VStack with LazyVStack in the chat message list so only visible messages are rendered, instead of all N messages on every re-render
• Move the inputText draft-save onChange observer from ChatView into ChatInputBar, so that ChatView no longer re-renders on every keystroke

Root causes

1. Eager VStack — the message list used VStack, which renders every message upfront regardless of visibility. With a long chat, any re-render (including keystrokes) caused SwiftUI to evaluate the full message list.

2. ChatView observed inputText — .onChange(of: viewModel.inputText) in ChatView.body registered ChatView as an observer of inputText. Every keystroke invalidated ChatView.body, triggering a full re-evaluation of all those message views. With a long chat, this was proportionally more expensive.

Test plan

• Type in the input bar with a long chat open — should feel responsive with no lag
• Scroll up through history — older messages load on demand correctly
• Send a message and verify streaming still auto-scrolls to bottom
• Draft auto-save still fires (500ms debounce) when typing
• Stop/interrupt still works during streaming

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