Auto-show What's New dialog on first launch after update

Track last_seen_version in session state. Show the What's New dialog
automatically when the version changes, but not on very first run
(where the welcome wizard is shown instead).

pixstrip-core/src/storage.rs

@@ -188,6 +188,7 @@ pub struct SessionState {
     pub metadata_mode: Option<String>,
     pub watermark_enabled: Option<bool>,
     pub rename_enabled: Option<bool>,
+    pub last_seen_version: Option<String>,
 }
 
 pub struct SessionStore {

pixstrip-gtk/src/app.rs

@@ -409,6 +409,32 @@ fn build_ui(app: &adw::Application) {
     window.present();
 
     crate::welcome::show_welcome_if_first_launch(&window);
+
+    // Auto-show What's New dialog on first launch after update
+    {
+        let current_version = env!("CARGO_PKG_VERSION").to_string();
+        let session = pixstrip_core::storage::SessionStore::new();
+        let sess = session.load().unwrap_or_default();
+        let last_seen = sess.last_seen_version.clone().unwrap_or_default();
+
+        if last_seen != current_version {
+            // Update stored version immediately
+            let mut updated_sess = sess;
+            updated_sess.last_seen_version = Some(current_version.clone());
+            let _ = session.save(&updated_sess);
+
+            // Only show dialog if this is not the very first run
+            // (welcome wizard handles first run)
+            let config_store = pixstrip_core::storage::ConfigStore::new();
+            let config = config_store.load().unwrap_or_default();
+            if config.first_run_complete && !last_seen.is_empty() {
+                let w = window.clone();
+                glib::idle_add_local_once(move || {
+                    show_whats_new_dialog(&w);
+                });
+            }
+        }
+    }
 }
 
 fn build_menu() -> gtk::gio::Menu {