feat(dialog): WidgetData setListItemColors / listItemColors

Add a per-item foreground-color channel to the WidgetData dialog
protocol so plugins can color source-list entries to match the curve
colors in the plot. The Filter Editor toolbox uses it to render the
source-curve list with the same colors as the plot.

- widget_data.hpp: setListItemColors(name, colors).
- widget_data_view.hpp: listItemColors(name) accessor mirrored on the
  host side so the dialog engine can read it back when rebuilding the
  Qt model.

ABI: additive, no field reorder, no struct shrink. Existing plugins
that do not set the new channel see a default-empty optional and
render with the inherited foreground color. No host or plugin
recompile needed.

Test plan
- Existing widget_data_view_test still passes.
- Filter Editor (downstream) consumes the new channel; manual smoke
  shows source-list items in plot colors.

Author: pabloinigoblasco

pj_plugins/dialog_protocol/include/pj_plugins/host/widget_data_view.hpp

@@ -75,6 +75,9 @@ class WidgetDataView {
   [[nodiscard]] std::optional<std::vector<std::string>> selectedItems(std::string_view name) const {
     return getStringArray(name, "selected_items");
   }
+  [[nodiscard]] std::optional<std::vector<std::string>> listItemColors(std::string_view name) const {
+    return getStringArray(name, "list_item_colors");
+  }
 
   // --- QTableWidget ---
   [[nodiscard]] std::optional<std::vector<std::string>> tableHeaders(std::string_view name) const {

pj_plugins/dialog_protocol/include/pj_plugins/sdk/widget_data.hpp

@@ -93,6 +93,14 @@ class WidgetData {
     return *this;
   }
 
+  // Set foreground colors for QListWidget items, parallel to the items vector
+  // set by setListItems. Each entry is a CSS color string (e.g. "#ff0000") or
+  // empty to use the default palette color. Size must match the items vector.
+  WidgetData& setListItemColors(std::string_view name, const std::vector<std::string>& colors) {
+    entry(name)["list_item_colors"] = colors;
+    return *this;
+  }
+
   // --- QTableWidget ---
   WidgetData& setTableHeaders(std::string_view name, const std::vector<std::string>& headers) {
     entry(name)["headers"] = headers;