Fix double-scaling by locking page layout to logical presentation

Pages computed layout from SDL_GetWindowSize() (real window
coordinates) while SDL_SetRenderLogicalPresentation() independently
remapped every draw call into a 1920x1080 space. The two coordinate
systems only agreed when the window happened to be exactly 1920x1080;
at any other size, layout coordinates were transformed a second time,
so content shrank/grew quadratically toward the top-left on resize or
on non-1080p monitors. Commenting the call out masked the bug by
making rendering 1:1 with the window, but then nothing scaled: fonts
and margins stayed fixed-pixel while backgrounds cover-fit, and HiDPI
displays drew at point resolution and relied on blurry compositor
upsampling.

Restore SDL_SetRenderLogicalPresentation(1920x1080, LETTERBOX) as the
single source of scaling, and route all twelve layout sites through a
new radiantwave::getLayoutSize() helper (src/logical_size.h) that
returns the renderer's logical size, so pages always lay out against
the design resolution regardless of window or monitor geometry. Add
SDL_WINDOW_HIGH_PIXEL_DENSITY so HiDPI displays render at native pixel
density instead of being upscaled by the OS. Cached per-page textures
become resolution-independent since layout no longer depends on window
size.

src/logical_size.h

@@ -0,0 +1,17 @@
+#pragma once
+
+#include <SDL3/SDL.h>
+
+namespace radiantwave {
+
+inline void getLayoutSize(SDL_Renderer* renderer, int* w, int* h)
+{
+    SDL_RendererLogicalPresentation mode = SDL_LOGICAL_PRESENTATION_DISABLED;
+    if (!SDL_GetRenderLogicalPresentation(renderer, w, h, &mode) ||
+        mode == SDL_LOGICAL_PRESENTATION_DISABLED || *w <= 0 || *h <= 0)
+    {
+        SDL_GetWindowSize(SDL_GetRenderWindow(renderer), w, h);
+    }
+}
+
+} // namespace radiantwave