Add nozzle flow variant & make some options multi-variant (#13712)

# Description

Port BBS per-extruder config variants toggle, thanks Bambu!

Also fix invalid num error when resetting multi-variant filament
overrides.

TODOs:

- [x] Make more configs multi-variant (such as speeds)
- [x] Add flow variant (normal/high flow) combo box
- [x] Add botton to sync config to other variant


# Screenshots/Recordings/Graphs

<img width="846" height="1494" alt="image"
src="https://github.com/user-attachments/assets/47abf053-f5b8-4c86-ac0b-c0323ed8b242"
/>

<img width="1478" height="1189" alt="707420e5be01c30af3e6ede1f3dc9b67"
src="https://github.com/user-attachments/assets/11268712-da65-42ab-9cb8-3cede7790a5c"
/>

<img width="1478" height="1189" alt="81d5d3877f4d5bf9dccd44d7f0b30aa7"
src="https://github.com/user-attachments/assets/cda5ea51-07c3-48fc-bfbb-1428dca14e26"
/>


## Tests

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Fix #10336

src/libslic3r/PresetBundle.cpp

@@ -3832,9 +3832,9 @@ void PresetBundle::update_filament_count()
         : filament_presets.back());
 }
 
-bool PresetBundle::support_different_extruders()
+bool PresetBundle::support_different_extruders() const
 {
-    Preset& printer_preset = this->printers.get_edited_preset();
+    const Preset& printer_preset = this->printers.get_edited_preset();
     int extruder_count;
     bool supported = printer_preset.config.support_different_extruders(extruder_count);