Merged from feature/interrupt-on-type branch.
Backend cancellation infrastructure:
- Added tokio watch channel to SessionState for cancellation signaling
- Implemented cancel_chat command
- Modified chat command to use tokio::select! for racing requests vs cancellation
- When cancelled, HTTP request to Ollama is dropped and returns early
- Added tokio dependency with sync feature
Story updates:
- Story 13: Updated to use Stop button pattern (industry standard)
- Story 18: Created placeholder for streaming responses
- Stories 15-17: Placeholders for future features
Frontend changes:
- Removed auto-interrupt on typing behavior (too confusing)
- Backend infrastructure ready for Stop button implementation
Note: Story 13 UI (Stop button) not yet implemented - backend ready
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