bug: Frontend has no request timeout or abort controller — long queries hang indefinitely

[QuantumByte-01]

Problem

sendMessage() in App.tsx has no timeout or AbortController. If the backend takes too long (e.g. Gemini is slow), the UI just spins forever with no way to cancel.

Also

• onKeyPress is deprecated in React — replace with onKeyDown
• checkApiHealth() is only called once on mount, never retried on reconnect
• clearChat() only clears frontend state — does not call /api/session/reset, so the backend still remembers the old conversation

Fix

• Add AbortController with a ~30s timeout to the fetch in sendMessage()
• Replace onKeyPress with onKeyDown
• Call /api/session/reset in clearChat()