feat: warm palette overhaul — cold zinc/teal to warm stone/amber

Replace the entire color system with a design-driven warm palette:

- Surface tokens: cold zinc grays → warm stone (#faf8f5 to #0f0e0d)
- Accent tokens: cold teal → warm amber (#fffbeb to #451a03)
- All bg-white → bg-surface-50 across templates (warm cream instead of pure white)
- Critical CSS: all hardcoded hex values updated to warm palette
- Prism code blocks: cold gray backgrounds → warm stone
- Pagefind search UI: blue buttons/links → amber interactive colors
- Dark mode: warm dark surfaces with amber accents throughout

Design system documented in .interface-design/system.md

Confab-Link: http://localhost:8080/sessions/bd3f7012-c703-47e9-bfe2-2ad04ce1842d

blog.njk

@@ -24,7 +24,7 @@ permalink: "blog/{% if pagination.pageNumber > 0 %}page/{{ pagination.pageNumber
   {% if paginatedPosts.length > 0 %}
   <filter-container oninit leave-url-alone>
     <div class="flex flex-wrap gap-3 mb-6">
-      <select data-filter-key="type" class="px-3 py-1.5 text-sm bg-white dark:bg-surface-800 border border-surface-200 dark:border-surface-700 rounded-lg">
+      <select data-filter-key="type" class="px-3 py-1.5 text-sm bg-surface-50 dark:bg-surface-800 border border-surface-200 dark:border-surface-700 rounded-lg">
         <option value="">All Types</option>
         <option value="article">Articles</option>
         <option value="note">Notes</option>