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/commands/chat.rs

@@ -30,6 +30,16 @@ pub async fn chat(
     config: ProviderConfig,
     state: State<'_, SessionState>,
 ) -> Result<Vec<Message>, String> {
+    // Reset cancel flag at start of new request
+    let _ = state.cancel_tx.send(false);
+
+    // Get a clone of the cancellation receiver
+    let mut cancel_rx = state.cancel_rx.clone();
+
+    // Mark the receiver as having seen the current (false) value
+    // This prevents changed() from firing immediately due to stale state
+    cancel_rx.borrow_and_update();
+
     // 1. Setup Provider
     let base_url = config
         .base_url
@@ -79,6 +89,11 @@ pub async fn chat(
     let mut turn_count = 0;
 
     loop {
+        // Check for cancellation at start of loop
+        if *cancel_rx.borrow() {
+            return Err("Chat cancelled by user".to_string());
+        }
+
         if turn_count >= MAX_TURNS {
             return Err("Max conversation turns reached.".to_string());
         }
@@ -86,7 +101,7 @@ pub async fn chat(
 
         // Call LLM with streaming
         let response = provider
-            .chat_stream(&app, &config.model, &current_history, tools)
+            .chat_stream(&app, &config.model, &current_history, tools, &mut cancel_rx)
             .await
             .map_err(|e| format!("LLM Error: {}", e))?;
 
@@ -108,6 +123,11 @@ pub async fn chat(
 
             // Execute Tools
             for call in tool_calls {
+                // Check for cancellation before executing each tool
+                if *cancel_rx.borrow() {
+                    return Err("Chat cancelled before tool execution".to_string());
+                }
+
                 let output = execute_tool(&call, &state).await;
 
                 let tool_msg = Message {
@@ -289,3 +309,9 @@ fn get_tool_definitions() -> Vec<ToolDefinition> {
         },
     ]
 }
+
+#[tauri::command]
+pub async fn cancel_chat(state: State<'_, SessionState>) -> Result<(), String> {
+    state.cancel_tx.send(true).map_err(|e| e.to_string())?;
+    Ok(())
+}