Fix Windows titlebar control overlap

[awtprod]

Summary

• Reserve the Windows caption-button area when Electron does not expose live Window Controls Overlay geometry.
• Keep app-owned titlebar actions, including the right-panel toggle, clear of minimize, maximize, and close.
• Preserve the existing exact geometry path whenever WCO is available.

Root cause

When the Window Controls Overlay API was unavailable, the renderer fell back to a 12px right inset even though Electron still displayed native Windows caption buttons. The right-panel toggle therefore occupied the native close-button hit area.

Impact

The desktop close button remains reachable, while the right-panel control stays visible and functional.

Validation

• vp fmt --check apps/web/src/index.css
• git diff --check
• Isolated integrated UI verification at 1400×900: the actual right-panel toggle ended at x=1244 and the simulated native caption area began at x=1262, leaving a 12px gap.
• Clicked the right-panel toggle and confirmed it changed from closed to pressed and rendered the surface chooser.