feat(dialog): add default_suffix to the save-file picker

docs/dialog-sdk-reference.md

@@ -57,7 +57,7 @@ For the full tutorial, see [dialog-plugin-guide.md](../pj_plugins/docs/dialog-pl
 | `setButtonIconNamed(name, icon_id)` | Set a button icon by id, resolved from the host's themed icon set (consistent tinting; unknown id → no icon) |
 | `setShortcut(name, key_sequence)` | Assign keyboard shortcut (e.g. `"Ctrl+A"`) |
 | `setFilePicker(name, text, filter, title)` | Turn into an **open** file picker (existing file) |
-| `setSaveFilePicker(name, text, filter, title)` | Turn into a **save-as** file picker (navigate + type a new filename); reports via `onFileSelected` |
+| `setSaveFilePicker(name, text, filter, title, default_suffix="")` | Turn into a **save-as** file picker (navigate + type a new filename); reports via `onFileSelected`. `default_suffix` is appended when the typed name has none |
 | `setFolderPicker(name, text, title)` | Turn into folder picker |
 
 ### QListWidget
@@ -150,7 +150,7 @@ Override these in your `DialogPluginTyped` subclass. Return `true` when state ch
 | `onValueChanged(name, int)` | QSpinBox | New integer value |
 | `onValueChanged(name, double)` | QDoubleSpinBox | New double value |
 | `onClicked(name)` | QPushButton | (no payload) |
-| `onFileSelected(name, path)` | QPushButton (file picker) | Selected file path |
+| `onFileSelected(name, path)` | QPushButton (file picker or save-file picker) | Selected file path |
 | `onFolderSelected(name, path)` | QPushButton (folder picker) | Selected folder path |
 | `onSelectionChanged(name, items)` | QListWidget, QTableWidget | Vector of selected item texts |
 | `onItemDoubleClicked(name, index)` | QListWidget, QTableWidget | Row index of double-clicked item |

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

@@ -322,6 +322,11 @@ class WidgetDataView {
     return it != w->end() && it->is_string() && it->get<std::string>() == "save_file_picker";
   }
 
+  /// The extension appended to a save-file picker's typed name when it has none.
+  [[nodiscard]] std::optional<std::string> saveFilePickerDefaultSuffix(std::string_view name) const {
+    return getString(name, "default_suffix");
+  }
+
   [[nodiscard]] std::optional<std::string> filePickerFilter(std::string_view name) const {
     return getString(name, "filter");
   }

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

@@ -418,14 +418,17 @@ class WidgetData {
   /// (which opens an existing file), this lets the user navigate to a directory AND
   /// type a new filename, so a not-yet-existing rule/config file can be created. The
   /// chosen path is reported through the same onFileSelected(name, path) event — the
-  /// plugin distinguishes save from open by the widget name.
+  /// plugin distinguishes save from open by the widget name. `default_suffix` is
+  /// appended when the typed name carries no extension.
   WidgetData& setSaveFilePicker(
-      std::string_view name, std::string_view button_text, std::string_view filter, std::string_view title) {
+      std::string_view name, std::string_view button_text, std::string_view filter, std::string_view title,
+      std::string_view default_suffix = "") {
     auto& e = entry(name);
     e["button_text"] = button_text;
     e["action"] = "save_file_picker";
     e["filter"] = filter;
     e["title"] = title;
+    e["default_suffix"] = default_suffix;
     return *this;
   }
 

pj_plugins/docs/dialog-plugin-guide.md

@@ -354,6 +354,7 @@ work like polling a server for available topics.
 | QDoubleSpinBox | `setValue(double)` | `onValueChanged(name, double)` |
 | QPushButton | `setButtonText` | `onClicked(name)` |
 | QPushButton (file picker) | `setFilePicker` | `onFileSelected(name, path)` |
+| QPushButton (save-file picker) | `setSaveFilePicker` | `onFileSelected(name, path)` |
 | QPushButton (folder picker) | `setFolderPicker` | `onFolderSelected(name, path)` |
 | QLabel | `setLabel` | (none — display only) |
 | QListWidget | `setListItems`, `setSelectedItems` | `onSelectionChanged(name, items)`, `onItemDoubleClicked(name, index)` |
@@ -457,6 +458,28 @@ bool onFileSelected(std::string_view name, std::string_view path) override {
 }
 ```
 
+### Save-file picker
+
+For an *export* button, use `setSaveFilePicker()` instead. The host shows a
+native save dialog (the user types a name and picks a location) and delivers the
+chosen path through the same `onFileSelected()` handler — distinguish the button
+by its `objectName`. The optional `default_suffix` is appended when the typed
+name carries no extension.
+
+```cpp
+// In widget_data():
+wd.setSaveFilePicker("export_btn", "Export...", "*.json", "Export Library", "json");
+
+// Same handler as the file picker, routed by name:
+bool onFileSelected(std::string_view name, std::string_view path) override {
+  if (name == "export_btn") {
+    writeLibraryTo(path);
+    return true;
+  }
+  return false;
+}
+```
+
 ### Dynamic visibility and enabled state
 
 Control widget visibility and enabled state from `widget_data()` to build