[2.x] fix: restore text-overflow: ellipsis on long button labels

[imorland]

Summary

Restores text-overflow: ellipsis on long button labels — regressed in #4174 when display: flex; flex-direction: column; was added to .Button-label to stack helper text below the label.

Flex containers don't honor text-overflow: ellipsis on themselves, so labels stopped truncating and started overflowing their containers.

Fixes #4621

Changes

Button.less — moved truncation to the inner span:

• Removed overflow: hidden; text-overflow: ellipsis; from .Button-label.
• Added the same rules to a new .Button-labelText block — that's the <span> already rendered inside every .Button-label (Button.tsx:122).
• Added min-width: 0; to .Button-label so the flex chain can shrink below content size when its parent applies shrink force.

Dropdown.less — fix for the SplitDropdown case shown in the issue:

• The main-action button (.SplitDropdown-button) sits inside a .ButtonGroup, where .ButtonGroup > .Button { flex-shrink: 0 } prevents shrinking. Without overriding that, the button would still grow past the group's max-width: 100%.
• Added .SplitDropdown-button { min-width: 0; flex-shrink: 1; } scoped under .Dropdown--split, leaving every other ButtonGroup user (post controls, admin permission grids, ExtensionNavButton) unchanged.

Test plan

• Discussion controls SplitDropdown with a long Reply label (e.g. Polish language pack as a guest, or Reply, but much longer as in the issue) — main button truncates with …, dropdown toggle stays at fixed width.
• SplitDropdown with a short label — appearance unchanged.
• ExtensionNavButton (admin extension list with long extension name) — still truncates correctly via its existing white-space: nowrap rule.
• Other ButtonGroup uses (post controls, permission grids) — appearance unchanged.
• Subscription notification email button row, header dropdowns — sanity check no regressions.

Notes

• No JS changes; the markup <span class="Button-label"><span class="Button-labelText">…</span>…</span> already exists.
• Backward compatible — third-party themes that target .Button-label for text styling still work; only the overflow behavior moves to .Button-labelText.