Refactor graphics handling to utilize Arena for graphics sheets management

Update various editors to replace instances of GraphicsSheetManager with gfx::Arena for accessing graphics sheets. This change enhances memory management and performance by centralizing graphics data handling within the Arena class. Clean up related code for improved clarity and maintainability.

src/app/editor/sprite/sprite_editor.cc

@@ -2,6 +2,7 @@
 
 #include "app/core/platform/file_dialog.h"
 #include "app/editor/sprite/zsprite.h"
+#include "app/gfx/arena.h"
 #include "app/gui/icons.h"
 #include "app/gui/input.h"
 #include "app/zelda3/sprite/sprite.h"
@@ -180,9 +181,8 @@ void SpriteEditor::DrawCurrentSheets() {
     graphics_sheet_canvas_.DrawTileSelector(32);
     for (int i = 0; i < 8; i++) {
       graphics_sheet_canvas_.DrawBitmap(
-          GraphicsSheetManager::GetInstance().gfx_sheets().at(
-              current_sheets_[i]),
-          1, (i * 0x40) + 1, 2);
+          gfx::Arena::Get().gfx_sheets().at(current_sheets_[i]), 1,
+          (i * 0x40) + 1, 2);
     }
     graphics_sheet_canvas_.DrawGrid();
     graphics_sheet_canvas_.DrawOverlay();