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/llm/ollama.rs

@@ -47,6 +47,7 @@ impl OllamaProvider {
         model: &str,
         messages: &[Message],
         tools: &[ToolDefinition],
+        cancel_rx: &mut tokio::sync::watch::Receiver<bool>,
     ) -> Result<CompletionResponse, String> {
         let client = reqwest::Client::new();
         let url = format!("{}/api/chat", self.base_url.trim_end_matches('/'));
@@ -108,7 +109,29 @@ impl OllamaProvider {
         let mut accumulated_content = String::new();
         let mut final_tool_calls: Option<Vec<ToolCall>> = None;
 
-        while let Some(chunk_result) = stream.next().await {
+        loop {
+            // Check for cancellation
+            if *cancel_rx.borrow() {
+                return Err("Chat cancelled by user".to_string());
+            }
+
+            let chunk_result = tokio::select! {
+                chunk = stream.next() => {
+                    match chunk {
+                        Some(c) => c,
+                        None => break,
+                    }
+                }
+                _ = cancel_rx.changed() => {
+                    // changed() fires on any change, check if it's actually true
+                    if *cancel_rx.borrow() {
+                        return Err("Chat cancelled by user".to_string());
+                    } else {
+                        continue;
+                    }
+                }
+            };
+
             let chunk = chunk_result.map_err(|e| format!("Stream error: {}", e))?;
             buffer.push_str(&String::from_utf8_lossy(&chunk));