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.

localization/i18n/OrcaSlicer.pot

@@ -18475,6 +18475,24 @@ msgstr ""
 msgid "Whether this printer supports fast purge mode with optimized temperature and multiplier."
 msgstr ""
 
+msgid "Purge mode"
+msgstr ""
+
+msgid "Purge Mode Settings"
+msgstr ""
+
+msgid "Prime Saving"
+msgstr ""
+
+msgid "Perform full purging with speed and temperature transition for the best print quality."
+msgstr ""
+
+msgid "Uses optimized purge temperature, multiplier and stronger extrusion for faster purging. May cause slight color mixing in some extreme cases."
+msgstr ""
+
+msgid "Reduces prime waste and prints faster. May cause slight color mixing or small surface defects."
+msgstr ""
+
 msgid "Deretraction speed (extruder change)"
 msgstr ""