fix(email): keep a small gutter on plain-text emails on mobile

The <=640px rule zeroes .email-content-text horizontal padding, so
plain-text (prose) emails render flush against the viewport edge on
phones, which hurts readability. Use a reduced 0.75rem gutter instead
of 0 — still maximizes width for wide content but keeps text off the
screen edge.

app/globals.css

@@ -242,8 +242,8 @@ body {
 
 @media (max-width: 640px) {
   .email-content-text {
-    padding-left: 0;
-    padding-right: 0;
+    padding-left: 0.75rem;
+    padding-right: 0.75rem;
   }
 }