Add font family selector for watermark text

Font picker using GTK FontDialog/FontDialogButton lets users choose
any installed system font for text watermarks. The selected font family
is passed through the processing pipeline and used to find the matching
font file on disk.

pixstrip-gtk/src/steps/step_watermark.rs

@@ -61,7 +61,31 @@ pub fn build_watermark_page(state: &AppState) -> adw::NavigationPage {
         .adjustment(&gtk::Adjustment::new(cfg.watermark_font_size as f64, 8.0, 200.0, 1.0, 10.0, 0.0))
         .build();
 
+    // Font family picker
+    let font_row = adw::ActionRow::builder()
+        .title("Font Family")
+        .subtitle("Choose a typeface for the watermark text")
+        .build();
+
+    let font_dialog = gtk::FontDialog::builder()
+        .title("Choose Watermark Font")
+        .modal(true)
+        .build();
+    let font_button = gtk::FontDialogButton::builder()
+        .dialog(&font_dialog)
+        .valign(gtk::Align::Center)
+        .build();
+
+    // Set initial font if one was previously selected
+    if !cfg.watermark_font_family.is_empty() {
+        let desc = gtk::pango::FontDescription::from_string(&cfg.watermark_font_family);
+        font_button.set_font_desc(&desc);
+    }
+
+    font_row.add_suffix(&font_button);
+
     text_group.add(&text_row);
+    text_group.add(&font_row);
     text_group.add(&font_size_row);
     content.append(&text_group);
 
@@ -415,6 +439,16 @@ pub fn build_watermark_page(state: &AppState) -> adw::NavigationPage {
             jc.borrow_mut().watermark_font_size = row.value() as f32;
         });
     }
+    // Wire font family picker
+    {
+        let jc = state.job_config.clone();
+        font_button.connect_font_desc_notify(move |btn| {
+            let desc = btn.font_desc();
+            if let Some(family) = desc.family() {
+                jc.borrow_mut().watermark_font_family = family.to_string();
+            }
+        });
+    }
     // Wire position grid buttons
     for (i, btn) in buttons.iter().enumerate() {
         let jc = state.job_config.clone();