refactor: move tenant templates to internal/tenant

- Move internal/build/ to internal/tenant/
- Rename assets for clarity
- Add tenant-blog.js for shared blog functionality
- Update style.css with improved code block styling

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/tenant/assets/js/tenant-blog.js

@@ -0,0 +1,10 @@
+(function() {
+  'use strict';
+
+  const channel = new BroadcastChannel('writekit-studio');
+  channel.onmessage = function(event) {
+    if (event.data.type === 'settings-changed') {
+      location.reload();
+    }
+  };
+})();