Add rename template presets and watermark color picker

- Rename step: quick-fill buttons for common patterns (Date+Name,
  EXIF Date+Name, Sequential, Dimensions, Camera+Date, Web-safe)
- Watermark step: color picker in advanced options using ColorDialogButton
- Add watermark_color field to JobConfig, wire through to core

pixstrip-gtk/src/steps/step_rename.rs

@@ -101,6 +101,44 @@ pub fn build_rename_page(state: &AppState) -> adw::NavigationPage {
         .text(&cfg.rename_template)
         .build();
 
+    // Preset template quick-fill buttons
+    let presets_flow = gtk::FlowBox::builder()
+        .selection_mode(gtk::SelectionMode::None)
+        .max_children_per_line(4)
+        .min_children_per_line(2)
+        .row_spacing(4)
+        .column_spacing(4)
+        .margin_top(4)
+        .margin_bottom(8)
+        .margin_start(12)
+        .margin_end(12)
+        .homogeneous(false)
+        .build();
+
+    let preset_templates = [
+        ("Date + Name", "{date}_{name}"),
+        ("EXIF Date + Name", "{exif_date}_{name}"),
+        ("Sequential", "{name}_{counter:4}"),
+        ("Dimensions", "{name}_{width}x{height}"),
+        ("Camera + Date", "{camera}_{exif_date}_{counter:3}"),
+        ("Web-safe", "{name}_web"),
+    ];
+
+    for (label, template) in &preset_templates {
+        let btn = gtk::Button::builder()
+            .label(*label)
+            .tooltip_text(*template)
+            .build();
+        btn.add_css_class("pill");
+
+        let tr = template_row.clone();
+        let tmpl = template.to_string();
+        btn.connect_clicked(move |_| {
+            tr.set_text(&tmpl);
+        });
+        presets_flow.append(&btn);
+    }
+
     let help_label = gtk::Label::builder()
         .label(
             "Available variables:\n\
@@ -145,6 +183,7 @@ pub fn build_rename_page(state: &AppState) -> adw::NavigationPage {
     regex_group.add(&replace_row);
 
     advanced_group.add(&template_row);
+    advanced_group.add(&presets_flow);
     advanced_group.add(&help_label);
     advanced_group.add(&case_row);
     content.append(&advanced_group);