feat(panels): wire the Effects section "+" to add a drop shadow

The property panel's 效果 header drew a "+" icon that did nothing.
Add a PropertyPanelAction::AddEffect, emit its hit rect over the
icon in the layout walker (no paint/layout change — the icon was
already painted), and dispatch it in both hosts to
Document::add_drop_shadow_to_selected. Clicking "+" now appends a
drop shadow that renders + persists.

crates/openpencil-shell-core/src/widgets/property_panel.rs

@@ -53,6 +53,9 @@ pub enum PropertyPanelAction {
     /// User clicked anywhere in the Export section — host queues
     /// `FileAction::ExportImage` so the picker dialog opens.
     OpenExportDialog,
+    /// User clicked the Effects section's "+" — host appends a
+    /// default drop shadow to the selected node.
+    AddEffect,
 }
 
 /// Per-NodeKind toggles for which property-panel sections render.

crates/openpencil-shell-core/src/widgets/property_panel_layout.rs

@@ -241,6 +241,14 @@ pub fn action_button_rects_with_fill_picker(
     }
     if visible.effects {
         // Mirrors paint_effects_section: header + 8 px filler.
+        // The header's "+" button (drawn by
+        // `paint_section_label_with_add` at the right edge) maps to
+        // an `AddEffect` action.
+        let plus = Rect {
+            origin: Point2D::new(x0 + w - PAD_X - 22.0, y),
+            size: Point2D::new(28.0, SECTION_HEADER_HEIGHT),
+        };
+        out.push((PropertyPanelAction::AddEffect, plus));
         y += SECTION_HEADER_HEIGHT;
         y += 8.0;
         y += SECTION_GAP;

crates/openpencil-shell-native/src/widget_host/property_dispatch.rs

@@ -46,6 +46,9 @@ impl WidgetHostNative {
                 self.document.ui.pending_file_action =
                     Some(openpencil_shell_core::document::FileAction::ExportImage);
             }
+            A::AddEffect => {
+                self.document.add_drop_shadow_to_selected();
+            }
         }
     }
 

crates/openpencil-shell-web/src/widget_host.rs

@@ -174,6 +174,9 @@ fn apply_property_action_impl(
         A::OpenColorPicker(target) => {
             let _ = document.open_color_picker(target, 0.0);
         }
+        A::AddEffect => {
+            document.add_drop_shadow_to_selected();
+        }
     }
 }