Fix newsletter issue pages rendering left-aligned on white; v2.3.1

The Resend email HTML uses align="left" on its outer wrapper table and a
white background, both fine inside a narrow inbox pane but broken in a
full-width browser tab: the content stuck to the left edge with a large
white field to its right. Override with CSS that centers the content and
matches the site's dark background.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

package.json

@@ -1,7 +1,7 @@
 {
   "name": "siteforge",
   "private": true,
-  "version": "2.3.0",
+  "version": "2.3.1",
   "type": "module",
   "scripts": {
     "dev": "vite",