[*] basic in-house cpp linting (#4039)

- add `#pragma once` to remainder files
- "correcter" defines (ANDROID), see https://groups.google.com/g/android-ndk/c/cf9_f1SLXls
- extra miscelly fixups

Signed-off-by: lizzie <lizzie@eden-emu.dev>

Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/4039
Reviewed-by: MaranBr <maranbr@eden-emu.dev>
Reviewed-by: crueter <crueter@eden-emu.dev>

src/frontend_common/data_manager.h

@@ -1,8 +1,7 @@
-// SPDX-FileCopyrightText: Copyright 2025 Eden Emulator Project
+// SPDX-FileCopyrightText: Copyright 2026 Eden Emulator Project
 // SPDX-License-Identifier: GPL-3.0-or-later
 
-#ifndef DATA_MANAGER_H
-#define DATA_MANAGER_H
+#pragma once
 
 #include "common/common_types.h"
 #include <string>
@@ -21,5 +20,3 @@ std::string ReadableBytesSize(u64 size) noexcept;
 u64 DataDirSize(DataDir dir);
 
 }; // namespace FrontendCommon::DataManager
-
-#endif // DATA_MANAGER_H