feat: a clear title bar moves the window with vertical tabs, not the sidebar

[Maxaubert]

Follow-up to #188 (#162), from the owner on 2026-10-11: "there should be a title bar to drag from not the sidebar. how about making it transparent".

• With vertical tabs and Show title bar off, a clear title bar: a 32 px row over the terminal only (from the sidebar's edge to the window's right), in the terminal's own ground, no line, no icon, no name. It moves the window and holds minimize, maximize and close (and the update chip, when there is one). The terminal starts right below it, so nothing runs under the buttons any more.
• The sidebar's empty space no longer moves the window; its tabs stay at the top and its buttons at the foot.
• With the sidebar collapsed, the clear bar spans the window.

App only. 0.39.4 -> 0.39.5.

Checks

• e2e verticalTabs (40 checks): one bar, the clear one, over the terminal only, no line, the terminal right below; the window buttons in it; Windows' own hit test: the clear bar answers HTCAPTION (2), the sidebar's empty space, its foot, a tab and the terminal answer HTCLIENT (1). titleBar passes.
• unit 1902/1902; typecheck and lint clean.

🤖 Generated with Claude Code

https://claude.ai/code/session_017XYVRPzvhLpPCr75uB6uaA