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/settings.cpp

@@ -10,6 +10,7 @@
 #include "email_address_page.h"
 #include "license_key_page.h"
 #include "date_format_page.h"
+#include "logical_size.h"
 #include <SDL3/SDL.h>
 #include <SDL3_image/SDL_image.h>
 #include <algorithm>
@@ -128,7 +129,7 @@ void Settings::update(float dt)
 void Settings::render(SDL_Renderer *renderer)
 {
     int winW, winH;
-    SDL_GetWindowSize(SDL_GetRenderWindow(renderer), &winW, &winH);
+    radiantwave::getLayoutSize(renderer, &winW, &winH);
     float fw = static_cast<float>(winW);
     float fh = static_cast<float>(winH);