Split device settings out of the per-slot save file

Deadzone (and future prefs like locale) now live in their own
settingsfile_t/settings.c, loaded eagerly at boot and saved immediately
on Apply, instead of inside savefile_t - a setting shouldn't reset or
diverge just because the player is on a different save slot, and this
also fixes settings changes not actually reaching disk until the next
full game Save.

PSP settings use a new plain sceIo path rather than sceUtilitySavedata,
since that dialog would flash its native icon on every settings tweak.
GameCube reuses the save system's existing memory card mount rather than
mounting it twice (settingsInit() now runs after saveInit()).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/duskpsp/input/inputpsp.c

@@ -6,7 +6,7 @@
  */
 
 #include "input/input.h"
-#include "save/save.h"
+#include "save/settings.h"
 
 // #define INPUT_PSP_GAMEPAD_BUTTON_ACCEPT INPUT_SDL2_GAMEPAD_BUTTON_CUSTOM
 // #define INPUT_PSP_GAMEPAD_BUTTON_CANCEL INPUT_SDL2_GAMEPAD_BUTTON_CUSTOM
@@ -95,5 +95,5 @@ errorret_t inputInitPSP(void) {
 }
 
 float_t inputGetDeadzoneSDL2(const inputbutton_t button) {
-  return saveGet(SAVE_ACTIVE_SLOT)->deadzone;
+  return settingsGet()->deadzone;
 }