Merge upstream main into the IMEX work

Upstream replaced MainFrame's fixed-index TabPosition enum with string-based
page ids ("feat: refactor notebook/tabs to be string based instead of fixed
index based"). The IMEX toolhead-visibility menu item was the only consumer of
that enum left on this branch, so its enable check now compares
m_tabpanel->GetSelectedPageName() against TAB_ID_PREVIEW -- the same form the
neighbouring upstream menu items use.

That mismatch is what broke CI: the branch built on its own, but the merge
commit CI builds no longer had TabPosition declared. No other conflicts.

666/666 tests pass in Release.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/libslic3r/PrintConfig.cpp

@@ -3488,9 +3488,8 @@ void PrintConfigDef::init_fff_params()
     def = this->add("sparse_infill_smooth_factor", coPercent);
     def->label = L("Sparse infill smooth factor");
     def->category = L("Strength");
-    def->tooltip = L("Controls how strongly sparse infill corners are rounded. 0% keeps the original right-angle path, "
-                     "while 100% produces the largest possible curves between adjacent infill lines. "
-                     "Currently applies only to the Hilbert Curve.");
+    def->tooltip = L("Controls how strongly sparse infill corners are rounded. 0% keeps the original sharp path, "
+                     "while 100% produces the largest possible curves between adjacent infill lines.");
     def->sidetext = "%";
     def->min = 0;
     def->max = 100;