fix(tabs): keep close button visible in squeezed tab strip

Closable tabs in a squeezed strip kept the 40px min-width of a plain
tab, so the close slot was clipped by overflow: hidden once the label
shrank. Give squeezed closable tabs a 64px min-width and pin the close
slot with flex: none so the X hit-target always stays visible.

Fixes #1740

src/ui/Tabs.module.css

@@ -140,6 +140,13 @@
   flex: 0 1 auto;
   min-width: 40px;
 }
+/* A closable tab keeps room for its close slot: only the label shrinks. */
+.squeeze .closable {
+  min-width: 64px;
+}
+.squeeze .closeSlot {
+  flex: none;
+}
 .squeeze .tabButton {
   flex: 1 1 auto;
   min-width: 0;