Story 13: Implement Stop button with backend cancellation

- Add tokio watch channel for cancellation signaling
- Implement cancel_chat command
- Add cancellation checks in streaming loop and before tool execution
- Stop button (■) replaces Send button (↑) during generation
- Preserve partial streaming content when cancelled
- Clean UX: no error messages on cancellation
- Backend properly stops streaming and prevents tool execution

Closes Story 13

src-tauri/Cargo.lock

@@ -2084,6 +2084,7 @@ dependencies = [
  "tauri-plugin-dialog",
  "tauri-plugin-opener",
  "tauri-plugin-store",
+ "tokio",
  "uuid",
  "walkdir",
 ]