Put the mail card on M3's corner scale and its header at title-lg

Plan step 19, containment.md C-27 and C-28. `.inner-body` was 24px, a
value not on M3's 0/4/8/12/16/20/28/32/48 scale; it is 28px, the
extra-large corner of the dialog and bottom-sheet surface it stands in
for. `.header a` is title-large at weight 400, which its own section
comment and the `h2` under it already said.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Qwx5USif3wFFmxtHg5U1g9

resources/views/mail/theme.blade.php

@@ -203,7 +203,7 @@ img {
     width: 100%;
 }
 
-/* Header: the app name in title-lg, or the configured logo */
+/* Header: the app name in title-lg (22/28 at weight 400), or the configured logo */
 
 .header {
     padding: 32px 0 24px;
@@ -213,7 +213,7 @@ img {
 .header a {
     color: {{ $role['on-surface'] }};
     font-size: 22px;
-    font-weight: 500;
+    font-weight: 400;
     letter-spacing: 0;
     line-height: 28px;
     text-decoration: none;
@@ -226,7 +226,7 @@ img {
     margin: 0 auto;
 }
 
-/* Body: the card, separated from the page by tone alone */
+/* Body: the card, separated from the page by tone alone, on M3's extra-large corner */
 
 .body {
     -premailer-cellpadding: 0;
@@ -245,7 +245,7 @@ img {
     -premailer-width: 570px;
     background-color: {{ $card }};
     border: 0;
-    border-radius: 24px;
+    border-radius: 28px;
     margin: 0 auto;
     padding: 0;
     width: 570px;