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>

lib/berrypod_web/router.ex

@@ -40,6 +40,7 @@ defmodule BerrypodWeb.Router do
 
   pipeline :admin do
     plug :put_root_layout, html: {BerrypodWeb.Layouts, :admin_root}
+    plug BerrypodWeb.Plugs.LoadTheme
   end
 
   # Public storefront (root level)