[FEATURE] Add stop generation button to cancel ongoing AI response (#364)

[adikulkarni006]

Closes #364

🎯 Problem

When the AI is generating a long response (streaming), users have no way to stop it mid‑way. They must wait or refresh the page.

🚀 Solution Implemented

• Backend – No changes (streaming API already supports AbortSignal).
• Frontend
  • api.js: streamMessage now accepts an optional signal parameter and passes it to fetch.
  • App.jsx: added abortController and isStreaming state, a stopGeneration function, and passes both to ChatWindow.
  • ChatWindow.jsx: shows a red Stop button while streaming; clicking it calls stopGeneration and aborts the request.
  • After abort, the partial response remains and [Stopped by user] is appended.

🧪 Testing Performed

• ✅ Start a long streaming response → Stop button appears.
• ✅ Click Stop → generation stops immediately, partial message stays with [Stopped by user].
• ✅ No errors in console; abort does not affect other requests.
• ✅ Existing Send behaviour unchanged.

📁 Files Changed

File	Change
frontend/src/utils/api.js	Added signal parameter to streamMessage
frontend/src/App.jsx	Added abort controller, isStreaming, stopGeneration
frontend/src/components/ChatWindow.jsx	Added Stop button & props

🔥 Why Merge?

• ✅ Improves UX – users can cancel long responses.
• ✅ Clean, minimal implementation using standard AbortController.
• ✅ No breaking changes – works with both streaming and non‑streaming modes.

Ready for review.

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[adikulkarni006]

@imDarshanGK PR is ready, all checks green. Please merge when you get a chance. Thanks!

[imDarshanGK]

@adikulkarni006 resolve conflicts