feat: configurable post-graph section and dedicated /graph page

- Add posting-activity.njk section template with configurable years/limit
- Register posting-activity in homepage-section.njk dispatcher
- Move hardcoded post-graph into Tier 2 block (homepage builder controls its own)
- Add "View full history" link to /graph/ on both Tier 2 and section template
- Create /graph/ page showing all years with no limit

Confab-Link: http://localhost:8080/sessions/edb1b7b0-da66-4486-bd9c-d1cfa7553b88

graph.njk

@@ -0,0 +1,18 @@
+---
+layout: layouts/base.njk
+title: Posting Activity
+permalink: /graph/
+withSidebar: true
+---
+
+<h1 class="text-2xl sm:text-3xl font-bold text-surface-900 dark:text-surface-100 mb-6">Posting Activity</h1>
+
+<p class="text-surface-600 dark:text-surface-400 mb-8">
+  A contribution-style graph showing posting frequency across all years.
+</p>
+
+{% if collections.posts and collections.posts.length %}
+  {% postGraph collections.posts, { limit: 0 } %}
+{% else %}
+  <p class="text-surface-500 dark:text-surface-400">No posts found.</p>
+{% endif %}