Fix list deletion sync: SyncAction::DeleteList

- Added DeleteList variant to SyncAction enum
- Added ApiClient::delete_list() calling DELETE /users/@me/lists/{id}
- List deletion uses DeleteList action (not Delete/delete_task)
- Sync engine handles DeleteList calling api.delete_list()

src/infrastructure/api.rs

@@ -402,6 +402,31 @@ impl ApiClient {
         Ok(())
     }
 
+    pub async fn delete_list(&self, list_id: &str) -> Result<(), ApiError> {
+        let token = self.get_token().await?;
+
+        let url = format!(
+            "https://tasks.googleapis.com/tasks/v1/users/@me/lists/{}",
+            list_id
+        );
+
+        let resp = self
+            .client
+            .delete(&url)
+            .bearer_auth(&token)
+            .send()
+            .await
+            .map_err(|e| ApiError::Network(e.to_string()))?;
+
+        if !resp.status().is_success() {
+            let status = resp.status();
+            let body = resp.text().await.unwrap_or_default();
+            return Err(ApiError::Api(format!("Delete list failed: {} - {}", status, body)));
+        }
+
+        Ok(())
+    }
+
     pub async fn move_task(
         &self,
         list_id: &str,