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/app.rs

@@ -65,6 +65,7 @@ pub struct JobConfig {
     pub watermark_position: u32,
     pub watermark_opacity: f32,
     pub watermark_font_size: f32,
+    pub watermark_color: [u8; 4],
     pub watermark_use_image: bool,
     // Rename
     pub rename_enabled: bool,
@@ -312,6 +313,7 @@ fn build_ui(app: &adw::Application) {
             watermark_position: 8, // BottomRight
             watermark_opacity: 0.5,
             watermark_font_size: 24.0,
+            watermark_color: [255, 255, 255, 255],
             watermark_use_image: false,
             rename_enabled: if remember { sess_state.rename_enabled.unwrap_or(false) } else { false },
             rename_prefix: String::new(),
@@ -1495,7 +1497,7 @@ fn run_processing(_window: &adw::ApplicationWindow, ui: &WizardUi) {
                 position,
                 font_size: cfg.watermark_font_size,
                 opacity: cfg.watermark_opacity,
-                color: [255, 255, 255, 255],
+                color: cfg.watermark_color,
             });
         }
     }
@@ -2348,7 +2350,7 @@ fn build_preset_from_config(cfg: &JobConfig, name: &str) -> pixstrip_core::prese
                 position,
                 font_size: cfg.watermark_font_size,
                 opacity: cfg.watermark_opacity,
-                color: [255, 255, 255, 255],
+                color: cfg.watermark_color,
             })
         } else {
             None