cumulative layout shift fixes

- inject theme toggle placeholder instead of loading dynamically
- explicitly set sticky header height
- set `font-display: fallback`

components/page-header/Header.module.css

@@ -2,6 +2,7 @@
   position: sticky;
   top: 0;
   width: 100%;
+  height: 4.5em;
   padding: 0.7em 1.5em;
   border-bottom: 1px solid var(--kinda-light);
   background-color: var(--background-header);
@@ -108,6 +109,7 @@
 @media screen and (max-width: 768px) {
   .header {
     padding: 0.75em 1.25em;
+    height: 5.9em;
   }
 
   .left {