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.

resources/profiles/BBL/machine/Bambu Lab A2L 0.4 nozzle.json

@@ -87,6 +87,7 @@
 	"retract_lift_below": [
 		"326"
 	],
+	"support_fast_purge_mode": "1",
 	"support_object_skip_flush": "1",
 	"upward_compatible_machine": [
 		"Bambu Lab H2S 0.4 nozzle"

resources/profiles/BBL/machine/fdm_machine_common.json

@@ -141,6 +141,7 @@
 	"single_extruder_multi_material": "1",
 	"support_air_filtration": "0",
 	"support_chamber_temp_control": "0",
+	"support_fast_purge_mode": "0",
 	"support_object_skip_flush": "0",
 	"wipe": [
 		"1"