feat(projects): cancel an in-flight project chat or analysis (#101)

[Hustler490]

Closes #101.

What

Lets users cancel a running project chat stream or analysis.

Backend (app/workspaces/routes.py)

api_project_chat_stream now accumulates the streamed tokens and:

• on client disconnect/cancel (GeneratorExit) closes the upstream provider generator and stops without persisting a partial assistant message;
• on a dropped socket (BrokenPipeError/ConnectionResetError) stops cleanly with no partial write;
• on success persists the accumulated reply (dropping the previous redundant second chat_with_project LLM call) and emits the done event.

The user message is still persisted up front.

Frontend (project.html + project.js)

• A Stop button appears while a chat stream or an analysis is running.
• Chat uses an AbortController on the stream fetch; analysis passes the same signal through the API helper. Aborts are treated as intentional (no error toast) and reset the UI.

Tests (tests/test_project_chat_cancel.py)

• Cancelling mid-stream (iterator.close()) persists no assistant message, keeps the user message.
• A completed stream persists the full streamed reply and emits done.

Acceptance criteria

• A cancel action aborts the stream and does not persist a partial assistant message.
• The SSE route stops yielding when the client disconnects.
• Non-streaming analyze has a cancellation path (client abort via AbortController).

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