Make the host per-monitor DPI aware and scale the ImGui overlay

The window sized itself to GetSystemMetrics(SM_CXSCREEN/CYSCREEN) but the
process was DPI-unaware, so on a scaled display (e.g. 4K @ 150%) Windows
handed us a virtualized resolution and bitmap-stretched the whole window up
to native -- softening the mirror, which is the tool's entire point.

Declare per-monitor-v2 awareness at startup so GetSystemMetrics/GetDpiForWindow
report true pixels. That alone would shrink the fixed-13px ImGui overlay to
crisp-but-tiny, so pair it with UI scaling: rebuild the default-font atlas at a
DPI-scaled SizePixels (crisp at the target size, unlike FontGlobalScale's
bitmap stretch) and ScaleAllSizes() the style. Net: same physical size as
before, now sharp.

- common/include/coop/dpi.hpp: pure DPI->scale math (uses USER_DEFAULT_SCREEN_DPI
  and a named kBaseFontPx, not bare 96/13 literals), with a zero fallback and
  clamping. Unit-tested by tests/dpi_test.cpp.
- imgui_layer: apply_dpi() at init from GetDpiForWindow; set_dpi() for runtime
  changes (rebuild atlas + reset/scale style + invalidate the DX11 font texture).
- d3d11_window: latch WM_DPICHANGED (honor the suggested rect), expose
  take_dpi_change(); main loop polls it and calls imgui.set_dpi() between frames.

The DPI math is unit-tested; the actual awareness + font rasterization + live
WM_DPICHANGED rescale are verified by hand.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

host/src/main.cpp

@@ -387,6 +387,13 @@ int run()
 
 	while (window.pump_messages())
 	{
+		// Rescale the overlay if the window changed DPI (moved monitors, or the display scale changed).
+		// pump_messages() latches the new DPI; apply it here, outside any in-progress ImGui frame.
+		if (unsigned new_dpi = 0; window.take_dpi_change(new_dpi))
+		{
+			imgui.set_dpi(new_dpi);
+		}
+
 		// When the operator enabled "Sync flip to game frames" (Hooked source), pace the
 		// whole iteration to the game: wait for the next published frame before rendering,
 		// then present without vsync so the flip lands in lockstep with the game.
@@ -539,6 +546,11 @@ int run()
 
 int WINAPI wWinMain(HINSTANCE, HINSTANCE, LPWSTR, int)
 {
+	// Per-monitor DPI awareness (v2): make GetSystemMetrics / GetDpiForWindow report true pixels so the
+	// mirror renders at the display's native resolution instead of a virtualized one that Windows would
+	// bitmap-stretch (softening the image). We scale the ImGui overlay ourselves to stay readable. Must
+	// run before any window is created.
+	SetProcessDpiAwarenessContext(DPI_AWARENESS_CONTEXT_PER_MONITOR_AWARE_V2);
 	// WGC requires an initialized apartment; multi-threaded suits the
 	// free-threaded frame pool.
 	winrt::init_apartment(winrt::apartment_type::multi_threaded);