Fix message ID assignment and state update in App.tsx

[dines-rl]

Summary

• Corrects the assignment of message IDs to ensure they are based on the current state length
• Updates the state update logic for user messages to use functional updates for consistency

Changes

src/App.tsx

• Modified user message creation to use the previous state length for ID assignment
• Changed setMessages call to use a functional update pattern when adding user messages
• Minor cleanup of whitespace in the async message handling block

Test plan

• Verify that user messages receive correct sequential IDs
• Confirm that messages are appended correctly without overwriting
• Test chat flow to ensure no regressions in message display or loading state

🌿 Generated by Terry

---

ℹ️ Tag @terragon-labs to ask questions and address PR feedback

📎 Task: https://www.terragonlabs.com/task/f2b9655d-ead2-4110-8c59-1c2e68e36489

[bolt-new-by-stackblitz]

Run & review this pull request in StackBlitz Codeflow.