fix: restore text-overflow: ellipsis on long button labels

Since #4174 added display: flex to .Button-label (so helper text could
stack below the label), text-overflow: ellipsis stopped working — flex
containers don't honor it on themselves.

Move overflow/text-overflow to the inner .Button-labelText span, and add
min-width: 0 to .Button-label so the flex chain can shrink below content
size when the parent applies shrink force.

For SplitDropdown, the main-action button sits inside a ButtonGroup
where .ButtonGroup > .Button has flex-shrink: 0 — so even with the above,
the button would still grow past the group's max-width. Override
flex-shrink/min-width on .SplitDropdown-button only, leaving other
ButtonGroup uses (post controls, admin permission grids) unchanged.

Fixes #4621

Author: imorland

framework/core/less/common/Button.less

@@ -272,10 +272,13 @@
 }
 .Button-label {
   line-height: inherit;
-  overflow: hidden;
-  text-overflow: ellipsis;
   display: flex;
   flex-direction: column;
+  min-width: 0;
+}
+.Button-labelText {
+  overflow: hidden;
+  text-overflow: ellipsis;
 }
 .Button-helperText {
   font-size: 0.73rem;

framework/core/less/common/Dropdown.less

@@ -119,6 +119,10 @@
 
 
 .Dropdown--split {
+  .SplitDropdown-button {
+    min-width: 0;
+    flex-shrink: 1;
+  }
   .Dropdown-toggle .Button-icon {
     display: none;
   }