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

css/tailwind.css

@@ -863,6 +863,20 @@
   }
 }
 
+/* Sparkline — inline SVG posting frequency chart */
+.sparkline {
+  width: 120px;
+  height: 28px;
+  display: block;
+}
+
+@media (min-width: 640px) {
+  .sparkline {
+    width: 180px;
+    height: 32px;
+  }
+}
+
 /* Save for Later buttons — hidden until auth confirmed */
 .save-later-btn {
   display: none;