feat: fix sparkline calculation, redesign with domain colors, add post-graph

- Fix sparkline downward trend by extrapolating partial current month
- Redesign sparkline SVG with gradient fill and responsive sizing
- Apply domain-specific colors (amber/rose/emerald/purple) via currentColor
- Add eleventy-plugin-post-graph for GitHub-style contribution grids
- Homepage: posting activity graph in Tier 2 default layout
- AI page (/ai/): stats dashboard + AI-involved posts graph injected via layout
- New filters: aiPosts (filter by AI level), aiStats (total/count/percentage)

Confab-Link: http://localhost:8080/sessions/956f4251-b4a9-4bc9-b214-53402ad1fe63

articles.njk

@@ -14,7 +14,7 @@ permalink: "articles/{% if pagination.pageNumber > 0 %}page/{{ pagination.pageNu
     <h1 class="text-2xl sm:text-3xl font-bold text-surface-900 dark:text-surface-100">Articles</h1>
     {% set sparklineSvg = collections.articles | postingFrequency %}
     {% if sparklineSvg %}
-    <span class="opacity-60 dark:opacity-40 text-surface-500 dark:text-surface-400">{{ sparklineSvg | safe }}</span>
+    <span class="text-amber-600 dark:text-amber-400">{{ sparklineSvg | safe }}</span>
     {% endif %}
   </div>
   <p class="text-surface-600 dark:text-surface-400 mb-6 sm:mb-8">