Scroller: use full window height on rotated/tall displays

Override the LETTERBOX logical presentation while the scroller is
active so it renders into the full physical window.  On a 1920x6480
rotated display the scroller now scrolls through all 6480 pixels of
vertical space instead of being confined to the 1920x1080 letterbox
region.  Text scale is unchanged (same font size, same pixel
rasterization); more lines are simply visible at once.

- onPush: save current logical presentation, switch to DISABLED with
  real window dimensions
- onPop: restore the saved LETTERBOX presentation
- resize handler in main.cpp: skip re-apply while scroller is active
  (it owns its own presentation and restores on pop)

src/scroller_page.h

@@ -115,4 +115,8 @@ private:
     float velocityDelay_ = 2.5f;
     int velocityState_ = 0;
     float velocityTimer_ = 0.0f;
+
+    int savedLogicalW_ = 0;
+    int savedLogicalH_ = 0;
+    SDL_RendererLogicalPresentation savedMode_ = SDL_LOGICAL_PRESENTATION_DISABLED;
 };