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/components/layouts/admin_root.html.heex

@@ -7,12 +7,23 @@
     <.live_title default="Admin" suffix=" · Berrypod">
       {assigns[:page_title]}
     </.live_title>
+    <!-- Preload critical fonts for the current typography preset -->
+    <%= for preload <- Berrypod.Theme.Fonts.preload_links(
+          @theme_settings.typography,
+          &BerrypodWeb.Endpoint.static_path/1
+        ) do %>
+      <link rel="preload" href={preload.href} as="font" type="font/woff2" crossorigin />
+    <% end %>
     <!-- Pre-declare layer order so shop reset < Tailwind base regardless of load order -->
     <style>
       @layer properties, reset, primitives, tokens, theme, base, components, layout, utilities, overrides;
     </style>
     <link phx-track-static rel="stylesheet" href={~p"/assets/css/admin.css"} />
     <link phx-track-static rel="stylesheet" href={~p"/assets/css/shop.css"} />
+    <!-- Generated theme CSS with @font-face declarations -->
+    <style id="theme-css">
+      <%= Phoenix.HTML.raw(@generated_css) %>
+    </style>
     <script defer phx-track-static src={~p"/assets/js/app.js"}>
     </script>
     <script>