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

_includes/components/sections/recent-posts.njk

@@ -39,7 +39,7 @@
         {% set borderClass = "border-l-[3px] border-l-surface-300 dark:border-l-surface-600" %}
       {% endif %}
 
-    <article class="h-entry p-4 bg-white dark:bg-surface-800 rounded-lg border border-surface-200 dark:border-surface-700 hover:border-accent-400 dark:hover:border-accent-600 transition-colors {{ borderClass }}">
+    <article class="h-entry p-4 bg-surface-50 dark:bg-surface-800 rounded-lg border border-surface-200 dark:border-surface-700 hover:border-accent-400 dark:hover:border-accent-600 transition-colors {{ borderClass }}">
 
       {% if likedUrl %}
       {# ── Like card ── #}