Move split controls to the title bar + title-bar/sidebar polish
- Split Right / Split Down move from the bottom bar to the top bar as
icon buttons (SplitRightIcon / SplitDownIcon) with hover tooltips, left
of the workspace selector, shown only in tiling mode. Wired through the
tiling handle (App.handleSplitRight/Down).
- Title bar: compact fixed-height buttons (size-5) with header padding so
hover highlights no longer reach the screen edge; tooltip + pointer
cursor on the sidebar toggle, split buttons, and workspace menu.
- Sidebar: fix asymmetric collapse animation — the open state now also
transitions width, so re-opening animates instead of snapping.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>