add WCAG-compliant input border contrast across all moods

New --t-border-input token per mood, all 3:1+ contrast against their
surface backgrounds (neutral #8c8c8c, warm #8a827a, cool #7a8591,
dark #707070). Used on admin inputs/selects/textareas and shop
themed-input/themed-select, with graceful fallback to --t-border-default.
Decorative borders on cards, dividers, panels are unchanged.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/berrypod/theme/css_generator.ex

@@ -72,6 +72,7 @@ defmodule Berrypod.Theme.CSSGenerator do
       --t-text-inverse: #ffffff;
       --t-border-default: #e5e5e5;
       --t-border-subtle: #f0f0f0;
+      --t-border-input: #8c8c8c;
     """
   end
 
@@ -88,6 +89,7 @@ defmodule Berrypod.Theme.CSSGenerator do
       --t-text-inverse: #ffffff;
       --t-border-default: #e7e0d8;
       --t-border-subtle: #f0ebe4;
+      --t-border-input: #8a827a;
     """
   end
 
@@ -104,6 +106,7 @@ defmodule Berrypod.Theme.CSSGenerator do
       --t-text-inverse: #ffffff;
       --t-border-default: #d4dce8;
       --t-border-subtle: #e8eff5;
+      --t-border-input: #7a8591;
     """
   end
 
@@ -120,6 +123,7 @@ defmodule Berrypod.Theme.CSSGenerator do
       --t-text-inverse: #171717;
       --t-border-default: #262626;
       --t-border-subtle: #1c1c1c;
+      --t-border-input: #707070;
     """
   end