[Installer][Client] Fix review findings: stale backup, empty reuse, relocate, portable warning

NSIS: wipe TEMP backup once per run to avoid resurrecting deleted files,
check data dir existence in relocate so dir-only trees are moved.

Client: never reuse a recent session for an empty source set, only fall
back to a covering session on copy failure, warn fresh portable installs
instead of staying silent.

cmake/NSIS.template.in

@@ -383,13 +383,15 @@ Function BackupUserDataBeforeUninstall
 ${If} $OldInstallDir == ""
     StrCpy $OldInstallDir $INSTDIR
 ${EndIf}
-; No wipe of $TEMP\CockatriceDataBackup here: %TEMP% survives across installer
+; %TEMP% survives across installer runs, so a previous run's safety copy would
-; runs, so the previous run's safety copy stays on disk until this run overwrites
+; otherwise merge into this run: CopyFiles merges/overwrites, resurrecting files
-; it. Stale content can never leak into this run because restore is gated on the
+; the user deleted since (e.g. deck A from run 1 reappearing alongside deck B
-; per-run $DataBackupDone / $LooseDecksBackupDone flags below, not on what happens
+; from run 2) or restoring stale content after a failed copy. Wipe once per
-; to be on disk. CopyFiles merges/overwrites, so calling this twice in sequence
+; installer run - $DataBackupDir is empty until the first backup of this run,
-; (32-bit then 64-bit uninstall) can only add safety, never delete the first
+; so the second call in sequence (32-bit then 64-bit uninstall) still merges.
-; call's copy.
+${If} $DataBackupDir == ""
+    RMDir /r "$TEMP\CockatriceDataBackup"
+${EndIf}
 ${If} ${FileExists} "$OldInstallDir\portable.dat"
 ${AndIf} ${FileExists} "$OldInstallDir\data"
     StrCpy $DataBackupDir "$TEMP\CockatriceDataBackup"
@@ -485,7 +487,7 @@ StrCmp $DataBackupDir "" done
 StrCmp $OldInstallDir "" done
 StrCmp $OldInstallDir $INSTDIR done
 ${If} $DataBackupDone == "1"
-${AndIf} ${FileExists} "$OldInstallDir\data\*.*"
+${AndIf} ${FileExists} "$OldInstallDir\data"
     !insertmacro CockLog "relocate: copying restored data $OldInstallDir\data -> $INSTDIR\data"
     CreateDirectory "$INSTDIR\data"
     CopyFiles /SILENT "$OldInstallDir\data\*.*" "$INSTDIR\data\"

cockatrice/src/interface/window_main.cpp

@@ -674,8 +674,12 @@ void MainWindow::warnAboutDataPathsInProgramDir()
             }
             backupSources.append(offender);
         }
+        // A fresh portable install has no data on disk yet, but the user still
+        // needs to know their data will live where the 3.1.0/3.1.1 uninstaller
+        // deletes it - so fall through to the warning with an empty backup
+        // instead of returning silently.
         if (backupSources.isEmpty()) {
-            return;
+            qCInfo(MainWindowLog) << "Portable install with no existing data files, warning without backup";
         }
         offenders.clear();
     } else {
@@ -718,7 +722,11 @@ void MainWindow::warnAboutDataPathsInProgramDir()
     bool recentUsable = false;
     if (!recentSession.isEmpty()) {
         recentLocations = UserDataBackup::listSessionBackupLocations(recentSession);
-        recentUsable = !recentLocations.isEmpty() && UserDataBackup::isBackupSessionComplete(recentSession) &&
+        // An empty source set must never reuse a session: sessionCoversSources is
+        // vacuously true for it, which would claim an unrelated recent backup
+        // covers paths that were never backed up.
+        recentUsable = !sourcesToBackUp.isEmpty() && !recentLocations.isEmpty() &&
+                       UserDataBackup::isBackupSessionComplete(recentSession) &&
                        UserDataBackup::sessionCoversSources(recentSession, sourcesToBackUp);
     }
     if (recentUsable) {
@@ -747,10 +755,13 @@ void MainWindow::warnAboutDataPathsInProgramDir()
             UserDataBackup::pruneOldBackupSessions(backupRoot, 5);
             if (backupLocations.isEmpty() && !recentLocations.isEmpty()) {
                 // Fresh copy failed entirely: fall back to the recent session so the
-                // dialog still points at something, flagged incomplete below.
+                // dialog still points at something, flagged incomplete below - but
-                sessionDir = recentSession;
+                // only when it actually covers the current sources.
-                backupLocations = recentLocations;
+                if (!recentSession.isEmpty() && UserDataBackup::sessionCoversSources(recentSession, sourcesToBackUp)) {
-                backupComplete = false;
+                    sessionDir = recentSession;
+                    backupLocations = recentLocations;
+                    backupComplete = false;
+                }
             }
         }
     }
@@ -788,6 +799,11 @@ void MainWindow::warnAboutDataPathsInProgramDir()
                          "before uninstalling or updating.\n\nReplays and downloaded pictures would not have been "
                          "included in that backup either.")
                           .arg(sizeInfo);
+        } else if (sourcesToBackUp.isEmpty()) {
+            message = tr("This portable install keeps its data inside the program directory, where the "
+                         "uninstallers shipped in Cockatrice 3.1.0/3.1.1 delete everything on uninstall or "
+                         "update.\n\nNone of your data files exist on disk yet, so there was nothing to back up. "
+                         "Files created there later are still at risk until you have updated once.");
         } else {
             message = tr("This portable install keeps its data inside the program directory, where the "
                          "uninstallers shipped in Cockatrice 3.1.0/3.1.1 delete everything on uninstall or "