Rename content slot to main, remove h1 from header for better

accessibility, and unify SVG fill color to #fff

Update BaseLayout to use a header <p> for the website title, rename the
main slot/id to
"main", and update all pages to match. Add CSS rule svg { fill:
currentColor }
and size header > p the same as h1. Fix a minor comment typo.

src/pages/contact.astro

@@ -146,7 +146,7 @@ const msgValue = pickValue("msg");
       }
     </style>
   </Fragment>
-  <Fragment slot="content">
+  <Fragment slot="main">
     <h2>Contact</h2>
     {
       (nextAction != "complete" && (