feat(gui): add purge mode selector dialog

The engine already implements prime_volume_mode (Default/Saving/Fast)
but nothing in the UI could set it, leaving prime-saving unreachable on
multi-sub-nozzle extruders and fast purge unreachable on printers that
support it.

- new PurgeModeDialog with selectable Standard/Fast or
  Standard/Prime Saving cards depending on printer capability
- "Purge mode" sidebar button next to Flushing volumes; opens the
  dialog and stores the choice in the project config
- printer preset-load gating: button shown only when the printer has
  multiple sub-nozzles per extruder or sets support_fast_purge_mode;
  stale project values the printer cannot honor reset to Default
- enable fast purge on A2L 0.4 (support_fast_purge_mode), explicit
  default 0 in the common machine base
- new dialog strings added to OrcaSlicer.pot

Printers without these capabilities never show the button and their
projects keep prime_volume_mode at Default, so slicing output is
unchanged.

src/slic3r/GUI/PurgeModeDialog.hpp

@@ -0,0 +1,69 @@
+#pragma once
+
+#include <wx/dialog.h>
+#include <wx/panel.h>
+
+#include "GUI_Utils.hpp"
+#include "libslic3r/PrintConfig.hpp"
+#include "Widgets/Label.hpp"
+
+class wxStaticText;
+class wxStaticBitmap;
+
+namespace Slic3r {
+
+namespace GUI {
+
+class PurgeModeBtnPanel : public wxPanel
+{
+public:
+    PurgeModeBtnPanel(wxWindow *parent, const wxString &label, const wxString &detail, const std::string &icon_path);
+    void Select(bool selected);
+
+protected:
+    void OnPaint(wxPaintEvent &event);
+
+private:
+    void OnEnterWindow(wxMouseEvent &event);
+    void OnLeaveWindow(wxMouseEvent &event);
+
+    void UpdateStatus();
+
+    wxBitmap icon;
+    wxBitmap check_icon;
+
+    wxStaticBitmap *m_btn;
+    wxStaticBitmap *m_check_btn;
+    wxStaticText   *m_label;
+    Label          *m_detail;
+    bool            m_hover{false};
+    bool            m_selected{false};
+};
+
+enum class PurgeModeDialogType {
+    MultiNozzle,
+    FastMode
+};
+
+class PurgeModeDialog : public DPIDialog
+{
+public:
+    PurgeModeDialog(wxWindow *parent, PurgeModeDialogType dialog_type = PurgeModeDialogType::MultiNozzle);
+
+    PrimeVolumeMode get_selected_mode() const { return m_selected_mode; }
+
+protected:
+    void on_dpi_changed(const wxRect &suggested_rect) override;
+
+private:
+    void select_option(PrimeVolumeMode mode);
+    void update_panel_selection();
+
+    PurgeModeBtnPanel  *m_standard_panel;
+    PurgeModeBtnPanel  *m_saving_panel;
+    PrimeVolumeMode     m_selected_mode;
+    PurgeModeDialogType m_dialog_type;
+};
+
+} // namespace GUI
+} // namespace Slic3r