fix(gui): scroll the Settings body and add a Defaults reset to the editor

The Settings screen grew past a short window once the Appearance section
landed, forcing a manual resize to reach the Save button. Wrap the body in a
vertical ScrollArea (header stays pinned), mirroring the Host screen. Also
add a '↺ Defaults' button to the right of Save in the theme editor that
resets the draft to the original Default Dark palette (previews live).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/gui/mod.rs

@@ -1010,6 +1010,13 @@ impl PixelPassApp {
             ui.heading("Settings");
         });
         ui.separator();
+        // Body scrolls; the header above stays pinned. The Appearance editor
+        // (13 colour rows + Save) overflows a short window otherwise — you'd
+        // have to resize the window to reach the Save button.
+        egui::ScrollArea::vertical().show(ui, |ui| self.settings_body(ui));
+    }
+
+    fn settings_body(&mut self, ui: &mut egui::Ui) {
         ui.add_space(4.0);
 
         let resp = ui.checkbox(
@@ -1149,6 +1156,13 @@ impl PixelPassApp {
             if ui.button("💾 Save").clicked() {
                 self.save_draft_theme();
             }
+            // Reset the draft to the original Default Dark palette. Previews
+            // live (the editor applies the draft each frame), so it snaps back
+            // immediately; Save persists it, Cancel discards.
+            if ui.button("↺ Defaults").clicked() {
+                self.theme.draft = theme::default_dark();
+                self.theme.status = Some("Reset to the Default Dark palette.".to_string());
+            }
             if ui.button("Cancel").clicked() {
                 self.cancel_theme_edit();
             }