Change bed temperature type to use highest temp by default (#12486)

## Summary
- Change default bed temperature type from "By First filament" to "By Highest Temp"
- Move `bed_temperature_formula` option from develop mode to advanced mode for better accessibility
- Relocate UI control from "Basic Information - Advanced" to "Multimaterial Setup" section where it's more relevant for multi-filament printing
Using the highest temperature of all printed filaments is generally safer for bed adhesion than using the first filament's temperature, especially in multi-material prints where different filaments may require different bed temperatures.

<img width="390" height="239" alt="Screenshot 2026-02-26 at 18 44 56" src="https://github.com/user-attachments/assets/0fe9f8c7-062a-4a7a-8ab3-c52df7e74b26" />

src/libslic3r/PrintConfig.cpp

@@ -2405,13 +2405,13 @@ void PrintConfigDef::init_fff_params()
     def = this->add("bed_temperature_formula", coEnum);
     def->label = L("Bed temperature type");
     def->tooltip = L("This option determines how the bed temperature is set during slicing: based on the temperature of the first filament or the highest temperature of the printed filaments.");
-    def->mode = comDevelop;
+    def->mode = comAdvanced;
     def->enum_keys_map = &ConfigOptionEnum<BedTempFormula>::get_enum_values();
     def->enum_values.push_back("by_first_filament");
     def->enum_values.push_back("by_highest_temp");
     def->enum_labels.push_back(L("By First filament"));
     def->enum_labels.push_back(L("By Highest Temp"));
-    def->set_default_value(new ConfigOptionEnum<BedTempFormula>(BedTempFormula::btfFirstFilament));
+    def->set_default_value(new ConfigOptionEnum<BedTempFormula>(BedTempFormula::btfHighestTemp));
 
     def = this->add("nozzle_flush_dataset", coInts);
     def->nullable = true;