Release @pi/ui 0.0.7: an empty checkbox can be seen

Its border was grey-100, 1.2:1 against white, so empty boxes all but
vanished in lists. It is grey-400 now (about 3.5:1, over the 3:1 WCAG
asks of controls), through --pi-theme-checkbox-border-color.

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

libs/ui/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@pi/ui",
-  "version": "0.0.6",
+  "version": "0.0.7",
   "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",