Fix CLI/GTK history timestamp format mismatch

CLI now stores Unix seconds (matching GTK) so age-based history
pruning works correctly. Human-readable formatting applied only
at display time in cmd_history and cmd_undo.

pixstrip-cli/src/main.rs

@@ -503,7 +503,7 @@ fn cmd_history() {
                 println!(
                     "{}. [{}] {} -> {} ({}/{} succeeded, {})",
                     i + 1,
-                    entry.timestamp,
+                    format_timestamp(&entry.timestamp),
                     entry.input_dir,
                     entry.output_dir,
                     entry.succeeded,
@@ -541,7 +541,7 @@ fn cmd_undo(count: usize) {
                 if entry.output_files.is_empty() {
                     println!(
                         "Batch from {} has no recorded output files - cannot undo",
-                        entry.timestamp
+                        format_timestamp(&entry.timestamp)
                     );
                     failed_entries.push(entry);
                     continue;
@@ -1041,14 +1041,20 @@ fn format_duration(ms: u64) -> String {
 }
 
 fn chrono_timestamp() -> String {
-    // Human-readable timestamp without chrono dependency
-    let now = std::time::SystemTime::now();
-    let secs = now
+    // Store as Unix seconds string (must match GTK format for pruning compatibility)
+    std::time::SystemTime::now()
         .duration_since(std::time::UNIX_EPOCH)
         .unwrap_or_default()
-        .as_secs();
+        .as_secs()
+        .to_string()
+}
+
+/// Format a Unix-seconds timestamp string as human-readable "YYYY-MM-DD HH:MM:SS"
+fn format_timestamp(ts: &str) -> String {
+    let Ok(secs) = ts.parse::<u64>() else {
+        return ts.to_string(); // already human-readable or unparseable - return as-is
+    };
 
-    // Convert to date/time components
     let days = secs / 86400;
     let time_secs = secs % 86400;
     let hours = time_secs / 3600;