Add inline split buttons next to New Tab

[khoi]

• add always-visible Split Vertically and Split Horizontally buttons next to New Tab in the terminal tab bar trailing accessories
• keep existing split actions wired to Ghostty bindings (new_split:right and new_split:down) and preserve canSplit disabled behavior
• keep the existing New Tab hover popover, and update split icons in both inline buttons and popover entries to square.split.2x1 / square.split.1x2 for consistency
• validate with make check, targeted xcodebuild test -only-testing:supacodeTests/TerminalTabManagerTests, and make build-app