fix: scale textarea line-height and add CSS variable default

Address review feedback on the FE-799 fix:

- WidgetTextarea now adds leading-normal so the textarea's line-height
  scales (unitless 1.5) with the configured font-size. Without it the
  inherited text-sm line-height of 1.25rem clipped multiline text at
  the larger end of the 8-24px range.
- Initialize --comfy-textarea-font-size to its default 10px on :root in
  the design-system stylesheet so isolated renders (Storybook, etc.)
  that do not run GraphView's watchEffect still get the documented
  default instead of an unset variable.

packages/design-system/src/css/style.css

@@ -120,6 +120,7 @@
   --comfy-topbar-height: 2.5rem;
   --workflow-tabs-height: 2.375rem;
   --comfy-input-bg: #222;
+  --comfy-textarea-font-size: 10px;
   --input-text: #ddd;
   --descrip-text: #999;
   --drag-text: #ccc;

src/renderer/extensions/vueNodes/widgets/components/WidgetTextarea.vue

@@ -22,7 +22,7 @@
       :class="
         cn(
           WidgetInputBaseClass,
-          'size-full resize-none text-(length:--comfy-textarea-font-size)',
+          'size-full resize-none text-(length:--comfy-textarea-font-size) leading-normal',
           !hideLayoutField && 'pt-5',
           // Avoid overflow-auto when idle to prevent per-textarea compositing layers.
           'overflow-hidden hover:overflow-auto focus:overflow-auto'