feat: turn sentinel dock tab into "+" new tab button

Instead of hiding the sentinel tab (which leaked space on macOS),
repurpose it as a visible "+" button that creates a new struct tab
on click. Compact 32px icon-only tab with pixel-perfect cross drawn
via fillRect. Skips context menu and middle-click. Always positioned
as the last tab in the group.

tests/test_rendered_view.cpp

@@ -327,7 +327,7 @@ private slots:
         QVERIFY(!code.contains("#pragma pack"));
         QVERIFY(!code.contains("#include <cstdint>"));
         QVERIFY(code.contains("#pragma once"));
-        QVERIFY(code.contains("struct TestStruct {"));
+        QVERIFY(code.contains("struct TestStruct"));
 
         // Load into rendered sci and verify colors survive
         QsciScintilla sci;