refactor: phases 5 & 6 — TransferService, Result<T>, RAII

Phase 5 — TransferService extraction:
- New nxst::TransferService owns all sender/receiver state, threads, and atomics
  (previously 11 file-statics across transfer_sender.cpp + transfer_receiver.cpp)
- startReceive() calls io::restore internally; UI never touches infra/fs or infra/net
- TitlesLayout routes all transfer ops through mainApp->transfer
- g_currentUId removed from global scope; TitlesLayout::InitTitles(AccountUid)
- MainApplication gains transfer member; layout refs renamed to snake_case
- userAppExit() updated to cancel via service

Phase 6 — Result<T> + RAII:
- include/nxst/domain/result.hpp: 85-line tagged-union Result<T,E> + Result<void,E>
- include/nxst/infra/fs/handles.hpp: FsFileSystemHandle (auto fsFsClose),
  FileHandle (auto fclose) — eliminates manual close on every error path
- io::backup / io::restore return nxst::Result<std::string> (was tuple<bool,Result,string>)
- new u8[] + malloc in copyFile/restore replaced with std::vector<u8>
- NACP save-creation extracted to createSaveIfNeeded() helper in io.cpp

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

include/nxst/infra/fs/io.hpp

@@ -26,27 +26,26 @@
 
 #pragma once
 #include <nxst/domain/account.hpp>
+#include <nxst/domain/result.hpp>
 #include <nxst/infra/fs/directory.hpp>
 #include <nxst/domain/title.hpp>
 #include <nxst/domain/util.hpp>
 #include <dirent.h>
 #include <switch.h>
 #include <sys/stat.h>
-#include <tuple>
 #include <unistd.h>
-#include <utility>
 
 #define BUFFER_SIZE 0x80000
 
 namespace io {
-    std::tuple<bool, Result, std::string> backup(size_t index, AccountUid uid);
-    std::tuple<bool, Result, std::string> restore(size_t index, AccountUid uid, size_t cellIndex, const std::string& nameFromCell);
+    nxst::Result<std::string> backup(size_t index, AccountUid uid);
+    nxst::Result<std::string> restore(size_t index, AccountUid uid, size_t cellIndex, const std::string& nameFromCell);
 
     Result copyDirectory(const std::string& srcPath, const std::string& dstPath);
-    void copyFile(const std::string& srcPath, const std::string& dstPath);
+    void   copyFile(const std::string& srcPath, const std::string& dstPath);
     Result createDirectory(const std::string& path);
     Result deleteFolderRecursively(const std::string& path);
-    bool directoryExists(const std::string& path);
-    bool fileExists(const std::string& path);
+    bool   directoryExists(const std::string& path);
+    bool   fileExists(const std::string& path);
 }