Subtitle: cap at 880px so it matches title+button row width

static/index.html

@@ -65,6 +65,9 @@
     font-size: 13px; font-weight: 300; line-height: 1.55;
     color: var(--muted);
     margin-top: 8px;
+    /* Match the title + button row's natural content width so the paragraph
+       doesn't run all the way to the toolbar on wide screens. */
+    max-width: 880px;
   }
   .subtitle a {
     color: var(--accent);