Add history pruning with configurable max entries and max days

- Add history_max_entries (default 50) and history_max_days (default 30)
  to AppConfig
- Add prune() method to HistoryStore that removes old entries by age
  and count limits
- Call prune after each history entry is added in the GUI
- Preserve history settings through settings dialog save/load cycle

pixstrip-gtk/src/app.rs

@@ -1629,6 +1629,11 @@ fn show_results(
         output_files,
     });
 
+    // Prune old history entries
+    let config_store = pixstrip_core::storage::ConfigStore::new();
+    let app_config = config_store.load().unwrap_or_default();
+    let _ = history.prune(app_config.history_max_entries, app_config.history_max_days);
+
     // Show toast
     let savings = if result.total_input_bytes > 0 {
         let pct =