style(settings): emphasize the save action

Give the settings save button a dedicated success-green background with black text and a matching lightened hover state while retaining focus and disabled behavior. Add a renderer regression test for the exact scoped class and color contract.

src/renderer/views/settings/SettingsView.tsx

@@ -71,7 +71,7 @@ export function SettingsContent({ model, actions }: SettingsViewProps): ReactEle
           <span aria-live="polite" className={`settings-save-state is-${model.saveState}`} role="status">{saveLabel}</span>
         </div>
         <button
-          className="settings-button settings-button-primary"
+          className="settings-button settings-button-primary settings-save-button"
           disabled={saveDisabled}
           onClick={actions.onSave}
           type="button"

src/renderer/views/settings/settings.css

@@ -478,6 +478,12 @@
   color: var(--ui-primary-text);
 }
 
+.settings-save-button {
+  border-color: var(--ui-success);
+  background: var(--ui-success);
+  color: #000000;
+}
+
 .settings-button-danger {
   border-color: color-mix(in srgb, var(--ui-danger) 70%, var(--ui-border));
   background: transparent;
@@ -496,6 +502,11 @@
   color: var(--ui-primary-text);
 }
 
+.settings-save-button:hover:not(:disabled) {
+  background: color-mix(in srgb, var(--ui-success) 86%, #ffffff);
+  color: #000000;
+}
+
 .settings-account-workspace {
   display: grid;
   grid-template-rows: auto auto minmax(0, 1fr);