Rename Fullscreen → Full Window; add true desktop Fullscreen

[vecnode]

What & why

The previous "Fullscreen" button only filled the app's content area (the WebView viewport), not the OS desktop. This splits the two behaviours.

• Full Window — the renamed previous button. Fills the WebView viewport via the Fullscreen API on the preview pane; the native window is untouched. Behaviour unchanged.
• Fullscreen (new) — does the same preview-pane fullscreen and drives the native OS window into borderless full-screen, so the sketch covers the whole desktop.

How the desktop fullscreen works

A single JS→C++ bridge: window.neoSetDesktopFullscreen(bool), bound in main.cpp. On Windows it strips the window frame (WS_CAPTION/WS_THICKFRAME/…) and stretches the window over the monitor (MONITORINFO + SetWindowPos), saving the prior style and WINDOWPLACEMENT to restore. On Linux it calls gtk_window_fullscreen / gtk_window_unfullscreen. The JS keeps using requestFullscreen() on .right-panel (so the sketch — not the editor/menus — is what's shown), then calls the bridge; Esc leaves element fullscreen and a fullscreenchange handler restores the native window.

Reviewer notes

• node --check passes; the Debug target builds and links the new Win32 calls; app launches cleanly with the binding registered.
• The DOM↔native fullscreen interaction in WebView2 can't be exercised headlessly here — worth a manual check: Run a sketch → Fullscreen should cover the whole desktop with the centred canvas on white, and Esc should restore the 1280×720 window. Full Window should fill just the app window.

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