feat: Backend cancellation support for interrupting model responses

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

src-tauri/Cargo.lock

@@ -2067,6 +2067,7 @@ dependencies = [
  "tauri-plugin-dialog",
  "tauri-plugin-opener",
  "tauri-plugin-store",
+ "tokio",
  "uuid",
  "walkdir",
 ]