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/settings.rs

@@ -271,6 +271,10 @@ pub fn build_settings_dialog() -> adw::PreferencesDialog {
         });
     }
 
+    // Preserve history settings from current config (not exposed in UI yet)
+    let hist_max_entries = config.history_max_entries;
+    let hist_max_days = config.history_max_days;
+
     // Save settings when the dialog closes
     dialog.connect_closed(move |_| {
         let new_config = AppConfig {
@@ -305,6 +309,8 @@ pub fn build_settings_dialog() -> adw::PreferencesDialog {
             high_contrast: contrast_row.is_active(),
             large_text: large_text_row.is_active(),
             reduced_motion: motion_row.is_active(),
+            history_max_entries: hist_max_entries,
+            history_max_days: hist_max_days,
         };
 
         let store = ConfigStore::new();