fix(ai): never route a pristine-starter canvas into direct-modify

A fresh from-scratch canvas holds only the blank starter frame, and that
frame ships selected. A whole-new-design request phrased as a bare noun
phrase (no page/screen noun, no create verb — e.g. "Luxury webapp for
managing barbershop clients") slips past both new-screen exemption gates,
and the selected starter makes selected_target_instruction true, so the
request was launched as a modify turn on a near-empty canvas — the model
then emits an unparseable fragment and the user sees "Could not parse
design nodes". Guard should_launch_direct_modify: a canvas that is only
the blank starter frame has nothing real to modify, so any request on it
is NEW (desktop parity of web_chat_standard's page_children_empty => New).

crates/op-host-desktop/src/chat_session_launch.rs

@@ -234,6 +234,16 @@ pub fn launch_if_pending(
 }
 
 fn should_launch_direct_modify(state: &EditorState, user_text: &str) -> bool {
+    // A pristine "from-scratch" canvas holds only the blank starter frame:
+    // there is nothing real to modify, so ANY design request on it is a NEW
+    // design, never a modify — even when the starter frame is selected and the
+    // prompt is a bare noun phrase the new-screen gates don't recognize
+    // (measured: "Luxury webapp for managing barbershop clients" on a fresh
+    // canvas fell into run_modify_turn → glm flat-nodes → empty `"`). This is
+    // the desktop parity of web_chat_standard's `page_children_empty => New`.
+    if active_page_is_blank_starter_frame(state) {
+        return false;
+    }
     // A whole-screen draw request ("继续画一下 search 页面") must reach the
     // design pipeline's new-frame route, never get hijacked into editing the
     // existing frame in place — even when it also trips the modify classifier.

crates/op-host-desktop/src/chat_session_launch_selection_tests.rs

@@ -35,6 +35,39 @@ fn state_with_selected_card() -> EditorState {
     state
 }
 
+fn state_with_selected_header() -> EditorState {
+    let mut state = EditorState::new();
+    state.active_children_mut().clear();
+    state.active_children_mut().push(frame(
+        "screen",
+        "Client Dashboard",
+        vec![frame("header", "Header", Vec::new())],
+    ));
+    state.set_single_selection(op_editor_core::NodeId::new("header"));
+    state
+}
+
+#[test]
+fn blank_starter_canvas_never_launches_modify() {
+    let state = EditorState::starter();
+
+    assert!(!state.selection.is_empty(), "starter frame is selected");
+    assert!(
+        !should_launch_direct_modify(&state, "Luxury webapp for managing barbershop clients"),
+        "fresh blank starter canvas has no real target to modify"
+    );
+}
+
+#[test]
+fn real_content_modify_request_still_launches_modify() {
+    let state = state_with_selected_header();
+
+    assert!(
+        should_launch_direct_modify(&state, "change the header color to blue"),
+        "selected real content + clear edit request should still update in place"
+    );
+}
+
 #[test]
 fn selection_with_keywordless_instruction_launches_direct_modify() {
     let state = state_with_selected_card();