[Installer][Client] Fix stale backup resurrection, batch session backups, extend coverage

NSIS: clear stale %TEMP% backup and only arm restore when something was copied, directory-existence checks, back up loose .dec decks, drop downloaded pics but keep pics\CUSTOM.

Helper: single session dir per run (ms timestamps), same-basename disambiguation, partial copies still report their location.

libcockatrice_utility/libcockatrice/utility/user_data_backup.h

@@ -17,6 +17,7 @@
 #define USER_DATA_BACKUP_H
 
 #include <QString>
+#include <QStringList>
 
 namespace UserDataBackup
 {
@@ -24,18 +25,45 @@ namespace UserDataBackup
 /**
  * @brief Recursively copies @p sourceDir into @p destDir, creating it as needed.
  * @returns true when every file was copied (or there was nothing to copy).
+ * Symlinks are copied best-effort via QFile::copy; a dangling link fails that entry only.
  */
 bool copyDirectoryRecursively(const QString &sourceDir, const QString &destDir);
 
+/**
+ * @brief Creates a fresh timestamped session directory under @p backupRoot.
+ * @param backupRoot directory OUTSIDE the install tree that will hold timestamped backups.
+ * @param sessionDirOut receives the created directory on success.
+ * @returns true on success. Millisecond timestamps plus a numeric suffix keep rapid
+ * successive calls (e.g. one per installer phase) from sharing a directory.
+ */
+bool createBackupSessionDir(const QString &backupRoot, QString *sessionDirOut);
+
 /**
  * @brief Copies an at-risk path (file or directory) to a timestamped folder under @p backupRoot.
  * @param sourcePath file or directory holding user data (must exist).
  * @param backupRoot directory OUTSIDE the install tree that will hold timestamped backups.
- * @param backupLocationOut receives the created backup file or directory on success.
- * @returns true on success, false when the source is missing or nothing could be copied.
+ * @param backupLocationOut receives the created backup file or directory whenever a target
+ * was created, even when the copy was only partial (i.e. also on failure with a partial copy).
+ * @returns true when the whole source was copied, false when the source is missing or
+ * any entry failed. A false return with a non-empty @p backupLocationOut means a partial
+ * backup exists at that location.
  */
 bool backupAtRiskPath(const QString &sourcePath, const QString &backupRoot, QString *backupLocationOut = nullptr);
 
+/**
+ * @brief Copies several at-risk paths into a single timestamped session directory.
+ * @param sourcePaths files or directories holding user data; missing entries are skipped.
+ * @param backupRoot directory OUTSIDE the install tree that will hold timestamped backups.
+ * @param backupLocationsOut receives one entry per source that produced a backup target
+ * (including partial copies).
+ * @param sessionDirOut optionally receives the shared session directory.
+ * @returns true when every existing source was copied completely.
+ */
+bool backupAtRiskPaths(const QStringList &sourcePaths,
+                       const QString &backupRoot,
+                       QStringList *backupLocationsOut,
+                       QString *sessionDirOut = nullptr);
+
 /**
  * @brief Default backup root guaranteed to live outside the install directory.
  */