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/Plater.hpp

@@ -208,6 +208,7 @@ public:
     // Orca
     static bool should_show_SEMM_buttons();
     void show_SEMM_buttons();
+    void enable_purge_mode_btn(bool enable);
     void update_dynamic_filament_list();
 
     PlaterPresetComboBox *  printer_combox();