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

package.json

@@ -21,6 +21,7 @@
     "@fontsource/inter": "^5.2.8",
     "@quasibit/eleventy-plugin-sitemap": "^2.2.0",
     "@resvg/resvg-js": "^2.6.2",
+    "@rknightuk/eleventy-plugin-post-graph": "^1.0.8",
     "@zachleat/filter-container": "^4.0.0",
     "@zachleat/table-saw": "^1.0.7",
     "alpinejs": "^3.15.8",