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>

server/routes/newsletter.js

@@ -76,6 +76,15 @@ function buildIssuePageHtml(issue) {
 <meta name="twitter:description" content="${description}"/>
 <meta name="twitter:image" content="${escapeHtml(ogImage)}"/>
 <link rel="canonical" href="${escapeHtml(canonical)}"/>
+<style>
+/* The source HTML is a Resend/React Email template built for a narrow email-client
+   viewport: its outer wrapper table uses align="left" (which browsers translate to
+   float: left) and relies on the surrounding white to be invisible in a ~600px inbox
+   pane. In a full-width browser tab that leaves a left-stuck column on a white field.
+   Force it to sit centered on the site's dark background instead. */
+html, body { background-color: #0a0a08 !important; margin: 0; padding: 0; }
+table[align="left"] { float: none !important; margin-left: auto !important; margin-right: auto !important; }
+</style>
 </head>`
 
   const ctaBlock = `