Fix Flow Ratio dialog shrinking after main window minimize (#13545)

FlowRateCalibrationDialog was missing the SetSizeHints call that every
other calibration dialog in calib_dlg.cpp uses (Cornering, Pa, Pa Pattern,
Max Volumetric Speed, Temperature, Retraction, VFA, Input Shaping). Without
it, the dialog has no enforced minimum size: on wxGTK, iconizing the main
window while the dialog is open triggers a layout/refresh that re-runs
Fit() with transient zero-sized children, collapsing the dialog to ~222x249
px with the OK button clipped and unreachable. The window manager then
honors the (incorrect) small geometry hint, so the user cannot resize it
back manually.

Two changes:

* Add v_sizer->SetSizeHints(this) after Fit() in the constructor, matching
  the pattern used by all other dialogs in this file. This locks in the
  correct minimum the first time Fit() runs, before any iconize event.

* Remove the redundant Fit() from on_dpi_changed. With SetSizeHints in
  place the WM enforces the minimum, but the unconditional Fit() on every
  DPI/refresh signal was the trigger for the shrink path; Refresh() alone
  is sufficient here.

Co-authored-by: Packerlschupfer <packerl@schupfer.at>

src/slic3r/GUI/calib_dlg.cpp

@@ -1518,6 +1518,7 @@ FlowRateCalibrationDialog::FlowRateCalibrationDialog(wxWindow* parent, wxWindowI
 
     Layout();
     Fit();
+    v_sizer->SetSizeHints(this);
 }
 
 FlowRateCalibrationDialog::~FlowRateCalibrationDialog() {
@@ -1540,7 +1541,6 @@ void FlowRateCalibrationDialog::on_start(wxCommandEvent& event) {
 
 void FlowRateCalibrationDialog::on_dpi_changed(const wxRect& suggested_rect) {
     this->Refresh();
-    Fit();
 }
 
 }} // namespace Slic3r::GUI