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/src/lib.rs

@@ -23,7 +23,8 @@ pub fn run() {
             commands::search::search_files,
             commands::shell::exec_shell,
             commands::chat::chat,
-            commands::chat::get_ollama_models
+            commands::chat::get_ollama_models,
+            commands::chat::cancel_chat
         ])
         .run(tauri::generate_context!())
         .expect("error while running tauri application");