Release @pi/ui 0.0.8: a ticked checkbox follows the theme's surface

Its box was filled with white, which glared in dark mode. It takes
--pi-theme-field-surface now, white when a theme leaves it out.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

libs/ui/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@pi/ui",
-  "version": "0.0.7",
+  "version": "0.0.8",
   "description": "Librería de componentes Angular standalone (botones, campos con Signal Forms, cards, charts, drawer/hub/modal, theming vía tokens CSS...).",
   "license": "MIT",
   "generators": "./generators.json",

libs/ui/src/lib/styles/common/vendors/_mat-checkbox.scss

@@ -141,7 +141,10 @@
           &:checked {
             & ~ .mdc-checkbox__background {
               border-color: var(--pi-color-primary);
-              background-color: var(--pi-color-white);
+              background-color: var(
+                --pi-theme-field-surface,
+                var(--pi-color-white)
+              );
             }
           }
           & ~ .mdc-checkbox__background {
@@ -168,7 +171,10 @@
             &:checked {
               & ~ .mdc-checkbox__background {
                 border-color: var(--pi-color-primary);
-                background-color: var(--pi-color-white);
+                background-color: var(
+                  --pi-theme-field-surface,
+                  var(--pi-color-white)
+                );
               }
             }
           }