[FEATURE] Add "Stop generation" button to cancel ongoing AI response

[adikulkarni006]

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

Expected behavior

• While AI is streaming, a red "Stop" button (■) appears next to the message or in the input area.
• Clicking it cancels the current generation.
• Abort the fetch request (using AbortController).
• The partial response remains as is, and a note "[Stopped by user]" is appended.

Technical hints

• Frontend: frontend/src/components/ChatWindow.jsx – manage an AbortController for the streaming request.
• Add state isGenerating to show/hide stop button.
• On stop, call abortController.abort() and update UI.

Labels: enhancement, good-first-issue, SSoC26