shell::Rename: report both rename + copy errors on fallback

Cherry-picked the two surviving improvements from stale branch
claude/fix-reported-issues-Iyt4s (issue #54). Rest of the branch's
content (Starfield data dir, My Games spam fix) was already
absorbed in main from a separate pass.

- Rename copy fallback: previously dropped the original rename
  errno and returned generic ERROR_ACCESS_DENIED with no
  context. Now bubbles "Rename failed: %1. Copy fallback failed: %2"
  with both QFile::errorString() values so the log line actually
  identifies what went wrong.
- formatSystemMessage: added an explicit comment block on why
  the codebase's Windows error macros (5=ACCESS_DENIED) must not
  be passed through strerror (5=EIO) on Linux.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

1 files changed, 16 insertions, 4 deletions

diff --git a/libs/uibase/src/utility.cpp b/libs/uibase/src/utility.cpp
index 4304561..ddd96e2 100644
--- a/libs/uibase/src/utility.cpp
+++ b/libs/uibase/src/utility.cpp
@@ -563,17 +563,24 @@ namespace shell
     }
 #endif
 
+    const QString renameError = QString::fromStdString(ec.message());
+
     if (copyAllowed) {
-      if (QFile::copy(srcPath, destPath)) {
+      QFile copyFile(srcPath);
+      if (copyFile.copy(destPath)) {
         QFile::remove(srcPath);
         return Result::makeSuccess();
       }
+      return Result::makeFailure(
+          ERROR_ACCESS_DENIED,
+          QObject::tr("Rename failed: %1. Copy fallback failed: %2")
+              .arg(renameError, copyFile.errorString()));
     }
 
     // Propagate the real errno text rather than a generic Windows code so the
     // log line actually reflects what failed.
     const int err = ec.value();
-    QString msg = QString::fromStdString(ec.message());
+    QString msg   = renameError;
     if (msg.isEmpty()) {
       msg = QString("rename failed: errno=%1").arg(err);
     }
@@ -1083,8 +1090,13 @@ std::wstring formatSystemMessage(DWORD id)
   std::wstring getMessage(DWORD id, HMODULE mod);
   return formatMessage(id, getMessage(id, 0));
 #else
-  // On Linux, map common error codes to strings or use strerror for errno values
-  if (id == 0) {
+  // The codebase uses Windows error macros (ERROR_ACCESS_DENIED, etc.) as
+  // internal error representation. On Linux, strerror(id) would interpret
+  // these as POSIX errno values — which overlap numerically but mean
+  // completely different things (e.g. Windows error 5 = ACCESS_DENIED,
+  // Linux errno 5 = EIO). Map known Windows codes explicitly instead.
+  switch (id) {
+  case 0:  // ERROR_SUCCESS
     return L"Success";
   }