style(pwa-install-dialog): tune content text color and margins

Set content color to #393939 and apply 36px horizontal margin to align
with the dialog's visual rhythm.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/app/chat/components/pwa-install-dialog.module.css

@@ -44,8 +44,8 @@
 .content {
   font-size: var(--font-size-lg, 16px);
   line-height: 1.5;
-  color: var(--color-text-secondary, #9e9e9e);
-  margin: 0 0 var(--spacing-xxl, 24px);
+  color: #393939;
+  margin: 0 36px var(--spacing-xxl, 24px);
 }
 
 .actions {