migrate accent colours from HSL to oklch, inject theme into admin

Phase 1: Replace hex_to_hsl with hex_to_oklch in CSSGenerator, output
--t-accent-l/c/h instead of --t-accent-h/s/l. All 46 HSL accent
references across theme-semantic.css, theme-layer2-attributes.css, and
shop/components.css replaced with oklch/color-mix equivalents. Dead
style*= attribute selectors for button variants replaced with proper
class-based selectors. Added color-scheme: light/dark to mood output.

Phase 2: Add LoadTheme plug to admin pipeline, extend AdminLayoutHook
with theme_settings and generated_css assigns, add font preloads and
generated CSS injection to admin_root.html.heex. No visual changes to
admin yet — .themed wrapper added in next phase.

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

test/berrypod_web/live/theme_css_consistency_test.exs

@@ -145,9 +145,9 @@ defmodule BerrypodWeb.ThemeCSSConsistencyTest do
       assert css =~ "--space-lg:"
 
       # Slider-controlled values
-      assert css =~ "--t-accent-h:"
-      assert css =~ "--t-accent-s:"
       assert css =~ "--t-accent-l:"
+      assert css =~ "--t-accent-c:"
+      assert css =~ "--t-accent-h:"
       assert css =~ "--t-font-size-scale:"
       assert css =~ "--t-heading-weight-override:"
     end