Fix uninitialized first_layer_time on CLI-sliced 3MF (#13429)

PartPlate::store_to_3mf_structure read first_layer_time from the indirect cali_bboxes_data struct,
which the GUI populates at Plater.cpp:10600 but the CLI never writes to. The result was uninitialized
memory leaking into slice_info.config

Read directly from get_slice_result()->initial_layer_time, which is populated by
GCodeProcessor::finalize() in both code paths and matches the pattern already used a few lines
above for gcode_prediction.

Also default-initialize PlateBBoxData::first_layer_time to 0.0f as a defense against any other consumer
reading it without an explicit write.

src/libslic3r/GCode/ThumbnailData.hpp

@@ -78,7 +78,7 @@ struct PlateBBoxData
     int first_extruder = 0;
     float nozzle_diameter = 0.4;
     std::string bed_type;
-    float first_layer_time;
+    float first_layer_time = 0.0f;
     // version 1: use view type ColorPrint (filament color)
     // version 2: use view type FilamentId (filament id)
     int version = 2;