fix: add default headers only to new workspaces on insert (#356)

crates/yaak-models/src/queries/workspaces.rs

@@ -65,7 +65,28 @@ impl<'a> DbContext<'a> {
     }
 
     pub fn upsert_workspace(&self, w: &Workspace, source: &UpdateSource) -> Result<Workspace> {
-        self.upsert(w, source)
+        let mut workspace = w.clone();
+
+        // Add default headers only for NEW workspaces (empty ID means insert, not update)
+        // This prevents re-adding headers if a user intentionally removes all headers
+        if workspace.id.is_empty() && workspace.headers.is_empty() {
+            workspace.headers = vec![
+                HttpRequestHeader {
+                    enabled: true,
+                    name: "User-Agent".to_string(),
+                    value: "yaak".to_string(),
+                    id: None,
+                },
+                HttpRequestHeader {
+                    enabled: true,
+                    name: "Accept".to_string(),
+                    value: "*/*".to_string(),
+                    id: None,
+                },
+            ];
+        }
+
+        self.upsert(&workspace, source)
     }
 
     pub fn resolve_auth_for_workspace(