refactor: phase 6 cleanup — RAII handles and io::restore split

- AccountProfileHandle RAII wrapper in handles.hpp; applied in
  account.cpp (getUser, iconPath) replacing manual accountProfileClose
- FILE* in iconPath replaced with existing FileHandle RAII wrapper
- io::restore split into clearSaveRoot + extractAndCommit static helpers;
  public signature unchanged
- sendAll/recvAll kept as bool — callers don't propagate the error string,
  Result<void> would add noise without benefit

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

include/nxst/infra/fs/handles.hpp

@@ -49,4 +49,23 @@ struct FileHandle {
     }
 };
 
+// RAII wrapper for AccountProfile — auto-closes on destruction.
+struct AccountProfileHandle {
+    AccountProfile profile{};
+    bool valid{false};
+
+    AccountProfileHandle() = default;
+    ~AccountProfileHandle() {
+        if (valid)
+            accountProfileClose(&profile);
+    }  // NOLINT(modernize-use-equals-default)
+
+    AccountProfileHandle(const AccountProfileHandle&) = delete;
+    AccountProfileHandle& operator=(const AccountProfileHandle&) = delete;
+
+    AccountProfile* get() {
+        return &profile;
+    }
+};
+
 }  // namespace nxst