Fix title bar buttons unclickable on Windows

[jettiandeng]

Problem

On Windows, every button in the title bar was dead: the expand-sidebar button did nothing, the language toggle felt stuck and never switched, and the Open / setup / UI-size / theme buttons had the same defect.

Root cause

gpui-kit's TitleBar marks its whole content strip as a native WindowControlArea::Drag region. On Windows, WM_NCHITTEST then answers HTCAPTION for every child of the bar (gpui's hit-test collects all ancestor hitboxes containing the cursor). A mouse press is treated as a window move: the click event never dispatches, and the unconsumed press falls through to DefWindowProc's modal move loop — hence the "laggy and ineffective" feel. macOS never takes this hit-test path, which is why the bug is Windows-only.

Fix

• Add .occlude() to the expand-sidebar, open-data, setup, ui-size and toggle-theme buttons: their hitboxes get BlockMouse behavior, the upward hitbox collection stops there, and Windows returns HTCLIENT — clicks dispatch normally while empty bar space still drags the window.
• Consume the left mouse press on the toggle-language button (prevent_default + stop_propagation), the same defence gpui-kit's own AppMenuBar uses.
• Add headless UI regression tests for the language toggle (the switch takes effect and the button relabels; the press does not bubble to ancestors), enabling gpui-kit's test-support feature in dev-dependencies.

Verification

• cargo check --bin ducklocal --tests: passes
• New language-toggle tests: 2/2 pass
• Existing cli (13) + lsp (8) tests: all pass, no regressions