Fixes a crash on Settings -> Close (preserve_AR=true) and the regression
where the resize cursor stopped appearing on the window borders.

Resize cursor: WM_SETCURSOR's non-fullscreen branch was breaking out of
the switch (added in #587 for fullscreen cursor hiding), which falls
through to "return 0" — telling Windows not to change the cursor. Defer
to DefWindowProc instead so it picks IDC_SIZEWE/etc. based on the
WM_NCHITTEST result. Drag-resize itself was working; just invisible.

Crash on close: SnapWindowTo16By9IfNeeded ran synchronously inside the
ImGui Close button handler. SetWindowPos -> WM_SIZE -> ResizeSwapChain
released RendererDX11's RTV while RenderIfNeeded still held the raw
pointer; the trailing OMSetRenderTargets dereferenced it. Now CloseDialog
posts a WM_APP message via PostMessageW and CDisplayDX11::WndProc runs
the snap from the message pump, after the render frame has returned.

CreateDisplayWindow: the work-area clamp now scales width and height
uniformly so the 16:9 design ratio is preserved when the requested size
doesn't fit. Independent per-axis shrinking previously produced ~1898x964
on a 1080p monitor at 150% DPI, which is what triggered the close-time
snap (and thus the crash).

Also enforce 16:9 on programmatic resizes (Aero snap, etc.) by posting
the deferred-snap message from WM_SIZE when preserve_AR is on. Added an
early-return to SnapWindowTo16By9IfNeeded so the handler is idempotent
and won't feedback-loop on its own SetWindowPos.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>